Hi,

a 512/256 will of course stay the same size. but if you use a 512/257 texture it will use as much memory as a 512/512 will. just make sure to stay in power of 2 at any time. you can also use 64/1024 if needed, as long as the single sides are a power of two you are fine.

btw, its better to ask then to create something wrong
this forum is a goldmine for knowledge and tricks!

cheers and have a nice one