Good questions.
Looking at last years results, it turns out that the top Men 35+ and women P/1/2/3 all finished within a minute of each other. The big difference is that the Women P/1/2/3 field had a whole 6 racers. They won't like it, but they're riding with the masters men 35+/45+ this year. If we get enough registrations, we can break them in to their own field.
Agreed, men Cat 3 and now riding with the Men Cat 4 and will have their own prizes.
We've never set hard minimums on the field sizes. For the last 5 years, after registration closes we go over registration numbers with the officials, and agree on the final fields/groupings for the race.
The maximum field sizes are simply the largest allowed by USACycling rules. Let me think about that...