Standard window sizes are designed using dimensions that typically fit new constructions. If your window frame can’t support a standard window size, a custom-size window may be cheaper.

The size of posts and beams is determined by how much weight ... They are usually 2x4s or 2x6s spaced 16 or 24 inches ...

You're probably most familiar with the 2x4, but that's not the only standard size out there. Other common sizes include 1x3, ...