Regarding how you know what unit size you're bidding on in an RSG auction...in an RSG, you are bidding to win the chance to select from available resorts/dates (someone correct me if I'm wrong, but this seems like the best way to explain this!). Before you bid, you can go to a page to check availability for your dates, so you know what resorts are available. When you click on the little bar that says "list available dates" under the resort that interests you, a list table will come up, listing the dates AND unit types available. On the far right of the table is a little bar that says "request", and you can request the date/unit type you want prior to bidding/winning an auction.
Apparently, they can't guarantee that the availability will still be there at the end of the auction, but I understand that if you win an auction and they can't give you what you'd requested, they may either upgrade you (someone posted they got the Vistana this way!) or refund your money.