Mary Ann, the man you spoke with yesterday (most likely Andris) is steering you wrong. Between gouging people on price, misrepresenting steel's abilities in ads and scaring people away from properly sized plates, I've gone to great lengths to encourage people to obtain their plate from someone other than Stoughton. If you don't mind spending about three times the price for steel, though, Stoughton isn't horrible, but, please, don't take their advice on sizing.
Please tell me that your oven is deeper than 14 1/2 inches. That would be soul crushing if you spent that much on an oven and only got a shelf that's 14.5" deep. You said this was your oven, right?http://www.ajmadison.com/cgi-bin/ajmadison/PODC302J.html
In the specs it talks about a usable Oven Interior Depth of 17 1/4". As I said earlier, I'm hoping that you have a little leeway and can squeeze in an 18" deep plate- or even a tiny bit deeper.
You also don't need 23" in the width. If your oven is 23" wide, then you'll want to shoot for around 20" on the width. An 18" x 20" x 1/2" plate, cut in half (two 9" x 20" x 1/2" pieces), will give you two 25 lb. pieces. Each will be manageable and the total weight will be no problem for your oven shelves. If you want to trim the weight a bit, there's nothing wrong with 18 x 19 (two 9 x 19 x 1/2 pieces).
Btw, make sure he doesn't round all the edges, as you'll want square edges where the plates meet.