buddy, click the tl button at the top (in amongst all ya tools for hide/grid size, etc), thisis the texture lock button, will help you in getting the textures right.
a player height is: 72 units
a door frame should be (on the inside): 96 units tall, 64 units wide, and 16 units deep/or thick
a window: needs to start at 32 units above floor level, and be 64 units tall (the same height as a door)
a step: 64 units wide (across), 16 units deep, and 8 units high, a step any higher will look wierd, it will be usable, but it does look crappy.
wall height: 128 units tall, or 112 units tall, walls should be 16 units thick, as this porvides moderate cover form bullets, and looks realistic.
that's about all you need, anything you want the player to be able to "look over" should be 32 units tall, this includes banisters, tables, etc
this is only a rough guide, all maps can be different, but the player size is correct, and the wall/door/window/table height dimensions are sound.