what is the optimal size for RSS feed?

I can't find answer anywhere: what is the best size for RSS feed (how many items shall I go back in history) for a large site? I have it currently limited to 1000 most recent items and 500 Kilobytes, but I am afraid it might still be to heavy for some RSS bots. I also understand if I only show last 10 items some updates may be missed between bot visits? Could please somebody elaborate how is it supposed to work and provide a guideline? Thanks

anybody?