With the jump to 64 bits, there is no reason to have only fixed, pregenerated galaxy sizes. Why not take the Minecraft / Elite approach? Procedurally generated galaxy could be essentially infinite (in practice it would still be limited by 64-bit precision, and your computer memory and CPU, but it could still be many times larger than galaxy wholly generated at the start of the game).
Minecraft world is 7000x larger than the surface of the Earth for example.