This link to the NH backhoe page might give you some idea: New Holland North America: Products
You can also download the loader specs from there. Good luck.
GREG,
FINDING a used backhoe is the real kicker. As far as how big can you go will depend on several factors, one being whether you want to plumb the backhoe hydraulics to your TC-40 or use a backhoe with an independent hydraulic oil pump and tank system which works off of your tractor's PTO.
There are several guys here that will probably check in and tell you about their Woods BH90-X Groundbreakers that they have attached to TC-35s, TC-40s and TC45s. These tractors all have the same hydraulic ratings of 9.8 GPM which is accepted by the Woods company as they also make subframe mounting kits for these NH models.
I have been looking all over the U.S. for a used BH90-X (not even caring if was red, orange or green in color) or a similar qualified Bradco for over a year now with no results. I also own a NH TC-40 and as I write this, my new Woods BH-90X Groundbreaker with a hydraulic thumb is on a freight truck heading toward me and should land this next Tuesday. From my research standpoint and what I want out of a backhoe (I did very little research on the offshore brands-I prefer my equipment to be as much as possible manufactured on U.S. turf), this is the strongest backhoe with a hydraulic thumb on the market today that will plumb directly to my tractor.
Greg, I'm going to toss in an "edit". Your post was placed in NH buying/pricing. Is your intention to only look for a NH backhoe or are you open for other brands? If NH only, please disregard my post. If you are open all brands I suggest you post in NH owning/operating.