I am Peyton "monitor", and I make maps for the ESV Mapmaking team. I play Protoss primarily, but I fool around with Zerg and Terran often too. Right now I practice in custom games at a low-mid masters level. My map Korhal Compound is in the Blizzard Ladder. I am also the organizer of Map of the Month here on TeamLiquid.
A lot of people on Team Liquid make maps. Many of the maps are quite good, too.
There will be two topics that I will cover as concisely as I can: Map Size and Proportions. Each of these need careful attention when creating a competitive melee map, because they can make or break a map. Once you get down the proportions, which comes with experience, you can make a well balanced map.
When I say 'Map Size', I mean Playable Map Bounds. There are three measurements that the editor gives you- yellow, blue, and grey. The yellow dotten lines indicate that the players vision cuts off there (except player camera sees a bit farther). The blue line indicates where buildings cut off- you can't build anything past this. The grey layer, only visible by clicking Map --> Map Bounds, lets you adjust the Full map size. In the bottom right corner of this screen, it tells you the dimensions of Playable and Full map sizes. I refer to Playable map sizes when I talk in this thread.
When you start a map, you don't need to know exactly how it will fit- by the end though, it needs to fit within certain boundaries, or guidelines. As in Brood War, maps that are excessively large or too small have inherent imbalances (anything above or below 128x96 in a two player map was almost always considered flawed). Starcraft II maps aren't fully matured yet, but they do follow similar size rules.
When creating a map, keep in mind that a standard two-player map needs to be smaller than a standard four-player map, because they contain different spawn and symmetry properties. Part of the magic of a two player map is that it is smaller than a four player map. It doesn't need to be as big as a four player map because it will only have two mains, and less of the map is symmetrical (two sections instead of four sections).
A two player map is ideally around a 136x114 map size. This will allow space for 10-12 expansions and comfortable space to maneuver in. In most cases, a map larger than this can lead to having too long of a distance between expansions, and it will discourage aggression and/or harassment. And conversely, too small of a map will make aggression too powerful.
A four player map should be sized about 144x144. This will give enough room for 16 expansions. In my opinion, a 12 base four player map that uses rotational symmetry can almost never work. The problem is that Starcraft II is balanced off of a close third expansion, but rush distances need to be quite long. If you only have 12 bases, either the third base ends up being too far away or the rush distance is too short. I could be proved wrong though.
The rules are occasionally bent in competitive play, but without a plethora of knowledge of the game, larger or smaller sizes will not work. Every map will have some problem if it is too big or too small, whether it be distances, void space, open space, or proportions.
Getting down your proportions takes knowledge and time. After you've had a lot of mapping experience, it will come more naturally- you'll be able to see how many gateways it takes to wall a choke, how many creep tumors it takes to connect the 3rd to 4th, how much vision the Xel'Naga Tower has, how many barracks fit in the main, etc.. You'll also get a feel for each of the race's preferred terrain as you play the game more.
Zerg players like to have wide open space. This is because their units have shorter range overall compared to Protoss and Terran, thus rely on getting surrounds on the opponent's army. Having too many chokes will make it impossible for Zerg to comfortable engage. Make sure you've got enough open space in the appropriate areas for the swarm to succeed.
Protoss and Terran both like tight spaces vs Zerg. In particular, Protoss likes corridors and Terran likes chokes. Both of their armies are primarily composed of long range units (stalkers, marauders, marines, tanks, colossi, etc.) which are better when balled up and only attacking at one angle. Protoss also has forcefields to decimate the battlefield in small corridors by cutting off the opponent. Make sure that your map has chokes dispersed appropriately so that Protoss and Terran can exploit them with good micro- but don't overdo it!
To have good proportions, ultimately you must balance each of these elements. Do your best to offer a balance of chokes, corridors, and open space for surrounds. Mastering space distribution can be very difficult, but will come in time.
When you create the main base (a base with a spawn in it) the most important thing is getting the size and shape right. The optimum size of a main can differ with the size of the natural-- so if you have a lot of space in the natural, you want less space in the main and vice versa. A typical main is nicely sized with space for 30-35 CC's. Make sure enough of this space is located between CC and ramp, otherwise it will be very awkward for Terran and Protoss to rally units out of their base.
With 30-35 CC's of space, each race is comfortable. Protoss and Terran will have good room for production and tech buildings. Zerg won't have too much space to cover with creep and scout for proxies/drops. It will also lead to the ideal distance between the main and natural of about 1-2 creep tumors.
Most players don't realize, but naturals don't need to be big. If your main is correctly sized, then your natural can be quite small. In Brood War, building placement was extremely careful because the areas were very small. Starcraft 2 hasn't reach the same level because building positioning doesn't matter quite as much, but making the natural too big can result in bad space allocation.
Starcraft II naturals need three primary things. Firstly, they need space to wall off. This means however many barracks are required to complete the wall, and some room behind them to put bunkers and a medium size army (same applies to Protoss and Zerg). Secondly you need some space to build a few production buildings. The most comfortable spot for this space is in between the natural's CC and the wall. Thirdly, you need a little space behind the mineral line for static defense, and possible space for proxies.
Thirds can vary a lot once you get into them. At the most basic level, you need space to build enough structures to produce off of 3base constantly and enough room for an almost 200/200 army. If there isn't room in the third, there needs to be a logical place to build and put your army somewhere else. Players often like to park their armies between their natural and third to defend from harassment at all bases. If there isn't a good spot to do this, make sure there is enough room at the third for a small army.
Take a look at the following maps for reference on this. They all do a extraordinary job with proportions.
[M](2) Sanshorn Mist by Superouman
[M](2) Alpha Stations by winpark
[M](2) Korhal Compound by monitor
When making maps, I ask that you please pay attention to your proportions and space allocation. Use the above maps for reference, you can find them on most servers. Check them out in-game, and good luck mapping!