I live in the new jersey and our floor contractor will install 6" boards, but they have to be glued, not just nailed. There is an extra charge for the glue of course. Hope that helps, and good luck!
BTW, the boards are oak, rift and quartered. Not sure whether type of wood makes a big difference.

As wide as you can find and pay for.

And wide lumber goes up in price a LOT.

Wider planks WILL have more movement from heating to cooling season, and have a higher chance of warping.

The link below is to Chapter 3 of the USDA 'Wood Engineering Handbook.'

It covers the movement of wood with variation in moisture content (and it WILL vary).

Figure 3-3 is very good at showing how cut wood changes shape depending on its original location in the tree.

Here is a link that might be useful: Wood Handbook, Chapter 3

Thanks so much for your replies. Brickeyee thank you for that link. I had a feeling that the guy wasn't quite right about being able to do it- I think he just didn't want to do it. I'm so glad to know that it is a possibility. I just need to figure out what style DH and I actually like and can agree on, and then I'll take photos to the flooring guy and tell him what we want.

The flooring guy was being circumspect in his statement. Flooring specialists (yes, I like the word) can come to your job site and assess your conditions and then recommend what can be done. Stability of installed wood flooring has much to do with subfloor type, moisture content of both it and the flooring, and ambient conditions within the home. Other posters have suggested the latter, when they say they maintain fairly constant humidity conditions in their homes.

You will need to find a flooring person willing to take on the job after he or she inspects your job site.

I think you also need to consider what you're asking the flooring specialists (sales, installation, whatever...) to do. Are you asking them to recommend/sell/install the flooring with a warranty or assurance you can come back to them if the wood moves?

I assume the original flooring guy that said he wouldn't install planks wider than 4.5" meant he wouldn't install them and back them with any kind assurance. Maybe he wouldn't install them under any circumstances because he's worried you'll drag his name through the mud if they cup/warp. Even if they inspect your site, how do they know you'll take the proper steps to control the humidity levels in both summer and winter? How do they know how "sensitive" you are to the inevitable gaps and minor cupping.

This was much the same discussion we had with our granite counter top dealer. If you want a perfect, uniform look, go with an engineered product. Natural stone has imperfections. Embrace it or pick something less natural. Same with wood flooring. The wider the plank, the more susceptible to movement in all directions.

It's easy to get someone on a message board to say they'll be fine with 10" wide planks. They likely understand the installation and maintenance requirements and, most importantly, don't have to worry about you suing them later if you're unhappy with your choice.

Real good answer.

I should have been clearer. We are building a new house and working with the guy at the builder's design center; the guy talked to us about wood flooring, tile (wall and floor), carpet, etc. Our contract cost breakdown stipulates 3" site finished oak floors is included in our price, and when we asked him about wider plank he didn't want to go much wider. He pushed us towards engineered floors instead. I didn't understand why he pushed toward engineered, especially because since that conversation I've seen many pictures online showing site finished oak in obviously wider than 3" strips.

Thanks for your replies! I feel more educated to go back to the design center and ask for clarification. :)