While the idea appeals to me, I do not believe there is an 'all purpose' nymph. I know of no pattern that scales adequately that I would use it in any situation (#22-8). Sure you can shoehorn any pattern into whatever size you like, but that is just a sign of stubbornness or inflexibility. I have several trusted patterns that I'll use in various situations, but no single pattern I would use in any situation.

Oddly enough, I would not give the same answer if you asked about an 'all purpose' dry fly...but that's another thread.