There are only 3 options for Transparency Grid size in Photoshop:
They do not conform to a pixel standard like you're thinking of them. They transmute with the display size. For example set to large on a 50px by 50px image when viewing it very large on my display gets:
Zoomed out without any other changes however gets us this:
As you can see the number of "squares" representing transparency is not constant.
It seems to me that you'd like the grid to act as a pixel grid to align or proportion artwork to. The intention of the checkerboard is to quickly show what part of your artboard is transparent. Useful if you're trying to make a cutout, or ensure that all areas are filled with block colour.
Can I suggest that the option to show the pixel grid might be more suitable for your purpose? View > Show > Grid (or cmd/ctrl - ') will show the pixel grid. If you're too zoomed out of course, you won't be able to see individual pixels, so make sure to zoom quite far in if you can see the lines pop up.
(similar question with pictures here: Photoshop CS6 - Can't find Show -> Pixel Grid)