I think the size is fine. It's the layout, items, and brushwork that need improvement. Of the 4 new maps, I think sand has the most potential, but...
FOR THE LOVE OF GOD QUIT SPAWNING EVERYONE WITH AUTOS
And yes, sand does have a bad spawn teamnumber. BSP has a handy "select by" tool that will let you select by key/value so you can quickly find spawns for different teams. As DT mentioned, you could also do some work on the terrain to make it look better. The flat 90 degree corners looks pretty bad with the sand texture. Also, arghrad has an option that lets you smooth out or phong shade the lighting on surfaces. If you set a value on a surface (but don't check the light flag), all the surfaces of that value will be blended together. I've actually got that set by default on the barrel textures (you may have noticed they don't look as blocky as they do on some older maps), and with the new BSP pack I'm working on, I'm setting it by default on some of the ground textures as well. The downside to this is that it can lead you to a MAX_PATCHES error a lot sooner. Oh, and a lot of barrel textures are misaligned, and there are some barrels part way in the wall and stuff. You should fix all that. I'd also get rid of the water. Not only does it look out of place in a desert-themed map, but it's annoying.
idea: needs better lighting (at first I didn't think it was lit at all, until I hit one small area with lighting). I'm not sure where you were going with the center area, but... it's weird, and you can get stuck. There's a lot of misaligned brushes. At the top of the hills, the hill is about 1 unit off leading to a very small vertical wall, which stops your forward momentum when running up it -- rather annoying. The 7-foot-tall sawhorses look funky, too. It might be an OK layout, though.
Beach: The lighting actually looks pretty good in this one, but the layout is blah. One path combined with all autos will lead to a very boring spray match. Also, when you get up on the towers, the r_speeds go through the roof.
Castle: meh... doesn't look much like a castle, and it's basically a box map. The lighting is way too bright.
Oh, and just for reference: 8 units is supposedly 1 foot (though that's a bit small, it's more like 9.3 units unless you want the player to be 7 feet tall). Many of the things in your map are extremely large. Most pb2 maps (and quake series maps in general) suffer from this, but it seems especially extreme in your maps. Some of the ladder rungs, for example, are almost 4 feet tall.