The focal length of the lens tends to indicate how much coverage the lens is capable of - longer lenses cover bigger negatives (some special lenses excepted).

Generally speaking, a 90mm lens is optimal for 6x7. Some 80mm lenses have enough coverage to be usable with 6x7. Many people use slightly longer lenses (e.g. 105mm) with 6x7, especially if they also shoot 6x9.

You can use longer lenses, but that will limit you to either smaller prints, or to using special arrangements like wall projection to get larger prints. As you probably understand, longer lenses result in lower magnification.

By the way, while it is true that you can use longer lenses than those that are optimal, the result may be that the lens is being asked to perform at a magnification that it is not optimized to work at. Good lenses can work well over a wide range, but they are best within a more narrow target range.

I use everything between 80mm and 150mm for my 6x7 negatives, because I do a fair number of small prints (e.g. postcards) from them. The longer lenses let me work with the enlarger head a reasonable distance from the paper. Otherwise, there can be a problem burning and dodging, or using my 11x14 easel.

Matt