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).

Either change the scaling on the blocks
(change it on the blocks directly, or by rightclicking ont the texture
to assign the texture in a specific scaling)

or change the texture-size in you favorite grafix programm,
erase it from the wad, and then import the rezised texture (with the exacly same name)

dont forget to save the wad!

well it's supposed to be a wood paneled floor but there is only one pannel of wood for the whole floor. What should i do to make it the floor look like it has a bunch of wood panels