The textures can be any size you want, as long as your hardware will allow it.

The reason people use certain sizes for textures is because at certain sizes the engine can use them better. These sizes are of the power of 2.

That means any binary numbers like 16*16, 32*32, 64*64, 128*128, 256*256, 512*512, 1024*1024...etc.

This saves memory because they don't have to be resized. Even if your texture is something like 258*280 the engine will go to the next power of 2 > being 512*512! Shows just how memory is wasted! LOL. If you use .dds, i think it is mandatory that you use a power of 2 texture size.

I don't recommend you go above 2048*2048. Only use that in the case that you maybe have a terrain that you really want to show details. But there are better methods to achieve that.

I don't think many GPUs can handle anything over 4096*4096 or even that, i haven't looked into it.