How can I make sure a cube create is exact amount of pixels?
is this possible?

for example, how can I be sure that my cube which I will place a 3d texture is 256x256 pixels. because my 3d texture will be 256x256x256 (rgba)


The size of the mesh really doesn’t matter when texturing, long as the texture coordinates are correct.

To answer your question though, you can make a cube that is about 256 pixels per face, it just requires a lot of math. An example would be like, if you have a viewport that is 512x512 pixels, and your cube face is supposed to be 256, you can draw a square that is 256x256 pixels. Then rotate the square, get the number of degrees you rotated it, then use that and a little bit of trig to figure out the positions of the vertices of the next face, and the next, and so on.