Hammer also shows the dimensions of selected brushes if ya didn't know.
I usually base my maps on blocks of 64 units. the player is a little smaller than 64 units wide and long, and 128 units high. My halls are usually 128 wide or 198 wide, which is 2 or 3 players wide. Its just a useful guide for me.
As far as objects go, i found it effective to proportion objects to inches. Ironically, 1 inch = 1 hammer unit. (despite what you might hear, the player is 73 units high. 72 inches = 6 feet)
Here is a copypaste from VERC, which has accurate dimension info. (thanks VERC!)
[quote]
Ground Obstacle Height
This is the maximum height, in units, an object can be while still allowing the player to walk over it.
* crouched: 18
* standing: 18
Overhead Obstacle Height
This is the minimum distance, in units, that can be between the ground and an object while still allowing the player to pass underneath.
* crouched: 37
* standing: 73
Minimum Path Width
This is the minimum width, in units, that can be between two objects while still allowing the player to pass between them.
* width: 33
Jump Height
This is the maximum height, in units, an object can be while still allowing the player to jump on top of it.
* crouched: 44
* walking: 44
* running crouch-jump: 62
Jump Span
This is the maximum distance, in units, that can be between two objects of the same heigh, while still allowing the player to jump from one object to the other.
* crouched: 99
* walking: 109
* running: 239
* running crouch-jump: 259
[/quote]
EDIT: Yeah, the player size in cs is noticebly smaller, making the map feel larger.