I would also go the smallest you can find....although there isn't really that much size variation for Ham on the bone. You can get half legs, which is what you will want.
They will cost roughly $40-70 depending on size etc.

However, ham will keep for a long time in the fridge still on the bone, just soak an old pillowcase in water and vinegar and ring it out, put your ham in and tie the end. You will need to re-wet the pillowcase every couple of days.

GL HTH!