Hi Manda. For a 20-25lb Turkey you would probably need a #9. Vintage #9 ovals from Wagner or Griswold can be quite spendy. They are also heavy add the bird/roast and other stuff and you have about 35 lb or so to move around. Just something to think about. They are also a good size so you should measure the cavity size of your oven.
I'm still looking for a marked CI Griswold #9, but not in a rush. If you want something to use and want to save some $$ and still get a quality piece consider vintage aluminum for this size roaster. You'll save a considerable amount of $$ and it will work really well. Vintage Griswold and Wagner oval roasters are pretty nice looking too. The cheapest alternative in something vintage (which I did) is a Wagner Magnalite. Beware there are new Magnelites out there that are made in China. Look for the older ones and you should be OK. You find Magnelite #9 PN4269-M sets in great shape on Ebay and they go in the 80-90 range depending on condition and trivet. I really like mine and use it fairly often, but not as much as the smaller ones which are all CI.
The cooler looking vintage Wagner and Griswold aluminum ovals in #9 are more expensive, than the old Magnalites but still not as much as the CI versions. These and the CI's are longer (handle to handle) than the mags too. So keep that in mind. But with either of them remember you can slide them in on an angle and the may fit.
Sorry - I'd jot down the RB and BB prices for you but - Christmas decorating has moved my reference books up to the 3rd floor and I'm too lazy to run for them right now. I'll update later.
Then again if you stay to 20lb as the top end a #7 might work just fine. They'll cost around $300 or so + or - $50 depending on condition and if a trivet is there. You really don't need a trivet but they are nice to have. Keeps the roast up out of the oil/fat while cooking. But you don't use them all the time. I really like my CI #7. You can save some money on CI ovals if you find one that has some pitting on the inside. It won't affect the cooking but will knock the price down a peg.
I've had a 20lb in my Wagner and it seemed to me that might have been able to fit a 25 in there.
Here's a handy size chart from one of the member's websites:
http://www.panman.com/sizecharts.htmlUpdat- had to run up stairs, so brought the books down
#9 BB For Griswold CI $300-400, but frankly they always seem to go for more than that in good condition the trivet is listed at $125
#9 RB for Wagner CI lists at $300-350, but always go for more at least on Ebay trivet lists at $100
#9 RB for Wagner AL lists at 75-125 for a #269 Magnalite 4269 lists at 100, but they go for less as I said
BB shows a #9 Griswold AL listing at $100-150. #A2
Can't say I've seen many (if any) aluminum Griswold ovals in #9 but have seen a good number of Wagners (on Ebay)