I have found amount of direct sun, drainage, and nutrients have a great effect on the size of rhizomes. Digging too early will reduce the size. Also, there is a variation of the rhizome size when comparing some of the moderns to some of the oldies. As someone said, freshness is more important than size unless they are the tiny new button size.
I happened to go to Wal-Mart to look for some marble chips when I saw they had two boxes of irises. They were bagged as 3 for $4.88. One was a solid blue and the other was a really nice dark pink/faded pink. I bought 2 bags of the pinks because one cannot go wrong at those prices. Turns out that the dark pink 'Cherub's Smile' is actually a flamingo pink. Actually, they could be any name and color--wait until they bloom.
The quality of the rhizomes leaves something to be desired but is typical of that which is usually sold at the big box stores. The broker is Van Zyerden.