I agree with Cantoo and wouldn't go higher than a 42. The question I have for you is, Do you have a 16 already or are you thinking of purchasing one?
If I can suggest the 20 as the minimum for commercial mowing for a few reasons, more hp is more efficient and can run more attatchments. The 20 can have a speed up kit attatched. The 20 is the minimum. If you can swing the 26EFI you wont believe the difference and you can run anything that Walker sells. I run a 26EFI with a 42" deck and it pulls sap out of the tree roots

If you need more info - e mail me. Good Luck