You can pretty much do what you like given the state of the building. As said, you can fit almost any size, anywhere.
Have a good think about where the hatch would be best, allowing for a decent hatch, ladder and headroom.
You just need to ensure that any cut joists are strengthened across the cut and made secure.
In your case it's easy because of no ceiling.
If you're going to board the loft out, consider getting some of the insulation and materials up there before plasterboarding the ceiling. May be a lot easier.