From Garry's Mod
Sets the size of a single tile. If a child panel is larger than this size, it will occupy several tiles.
If you are setting the size of the children properly then you probably don't need to change this.
The size of each tile. It is recommended you use 2n (16, 32, 64...) numbers, and those above 4, as numbers lower than this will result in many tiles being processed and therefore slow operation.