Too big? What do you mean by that? Taking up too large of space? Too big of texture pixels? Use the scale_x and scale_y values for the block's properties (right-click > properties), and set it from there. Anything below 0.25 for the texture scale is not recommended. It may seem like a 256x256 surface, but scaling it to 0.25 makes it seem more like a 1024x1024 surface. For anything higher, you're better off using models instead (let alone 50 times faster rendering and numerous other advantages).