The smallest you can go down to is 1 unit. If you have seen them smaller, you are most likely creating the map too small for the player. Try adding an info_player_start entity. It is a green box that represents the size of the player (for regular HL and CS) - info_evil_start or info_good_start for ESF. Albeit I doubt that it still needs to be smaller if it does, you cannot make it smaller. If you do (through the clip tool), it will cause a compile error. My suggestion, if it needs to be smaller, is that you don't use that block. If it is meant to be a connected part of a larger brush, try deleting the brushes and remaking the area without the small brushes. Remember that when you make brushes so small, it will seriously cause a decrease in FPS.