It's always good to match the size of the natural, so you have in fact answered your own question. Purists disagree, but right now, fishing eggs is "matching the hatch." The other comment is that a larger fly when viewed from further away by a fish, may in fact look the right size in his tiny brain and cause him to strike. I can see no harm in fishing eggs well downstream of any redds you may see. Lake run fish also seem exempt from any scrutiny by purists and non-purists alike.