Any size 5/8 to 7/8 and any grind will do if it has a good edge in general. I have one 8/8 and it is like shaving with a spatula to me. Smaller than 5/8 and they get a little dainty for my liking. If I had to narrow it down more, make mine a 6/8 extra or belied hollow. Goldilocks size for me and I love the sound of scraping toast when using a very finely ground razor.

Bob