You cannot. It works like this: The smallest size in Valve Hammer Editor is

*one unit* (This "one unit" happens to be the smallest grid size too). Each time you increase the grid size, it doubles. So the possible grid sizes are: 1, 2, 4, 8, 16, 32, 64, 128... The standard grid size is 64 units.

If you want a quarter of that, decrease the grid size twice- to 16.

If you want a fifth of that, you can't- 64 doesn't divide by 5. But you

__can__ decrease the grid size 4 times (to 4 units) and the make a 3x3x3 cube. This is roughly a fifth of 64.

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