marble floors in the bath with 1/4 space sanded or unsanded grout?

Answer 1

It doesn't really matter since the floor needs to be properly prepared and waterproofed prior to laying the marble. Once the grout is put down, the final step is to waterproof it also by "painting" a sealer over it. This needs to be redone every couple of years to keep moisture and stains out.
Of course theory says that sanded grout contains particles that cause channeling of water (much like soil in the garden), so some folks tend to go with unsanded grout on the premise that it won't create locks for water flow. Your choice

Good luck

Answer 2

You could also purchase epoxy grout. It's a little more expensive than unsanded grout, and requires mixing a few different ingredients. But it is worth the expense in the long run. Good luck with your project.