Let's say you win an auction that pushes your roster size to 54. You will not be able make any new bids until you reduce your roster to 53. Also, any active bids that you have when your roster reaches 54 are still valid, so it it is possible that your roster could grow to 55 or more. But no new bids, including counter offers, are allowed unless your roster is 53 or less. Also, before simulations start you will have to reduce your roster back to 53, or it will be done automatically.