Keeping things to size can be a problem, for instance with counter-strike the player models were lowered so this made some maps look a big large, like coke cans the size of boulders.

I find using bots is a good idea, there's jumbot for hl, and various other bots for mods, so have a mooch about with some bots in your map and see how big things are. You can also use hostages if you using cs, try putting them in the map somewhere and you can have them follow you ab00t

People are always asking how big one unit it in wc, im pretty confused about it myself, and I dont find it very helpful to know how big a unit is, because the grid can be on many size, 265, 64 etc, im not going to go into my views on it, as there silly, a player model in cs for example is 64x32.

halflife player sizes:

standing are 32x32x72
sat down / crouched 32x32x36
max step height of 17 units

(pretty useless isnt it :P > look at your grid values in the bottom status bar of wc)

Counter-strikes models are smaller than the average hl ones, so be aware of this, always check your sizes of your maps as you go, I once made a map without checking the sizes and it was huge! so take care! dod ones, firearms, etc all seem to be the same size, so its just cs thats tried to trick us :P