Longer rods make some line control easier ,so i would not recommend a rod less than 9 feet if you primarily nymph fish. Other than that choose a rod action that you enjoy. I will say one of the first rods i bought for myself as a kid was a very slow 9 foot Orvis fiberglass nymph rod( it was labeled as a nymph rod) but it was so soft and sloppy i sold it.
Probably the best recommendation is to use what have until you have enough experience to form your own opinion