Liek a previous poster, we use a Seal 210M Masterpiece too. So far only 8x10s on 14x17, but the 210M is big enough (about 18x22) to do a 16x20 on 22x28 in two passes. Once we're able to enlarge 5x7 and 8x10 negs, we plan to print and mount 20x24 but we're still undecided about board size and overmat.
We do our own overmats. We're dyi purists; we want to say of our work that we did it all ourselves. We use a Logan Simplex Plus 750, which goes to 40 inches. That's big enough to handle any reasonable board or overmat for 20x24, not to mention 16x20. No matter how good you get, you'll still get overcuts, but in our minds that's a desirable mark of hand work. Besides, esp. if you're contact printing, you're going to have some variation in print size. so being able to customize each overmat is often a good thing.