Keep in mind that when, or if, a fly recipe calls for specific amounts of materials (3 turns of hackle; 6 fibers for the tail; etc) these are for a specific size of fly. If you're making a fly in a different size, you must adjust the amounts of material to suit the size you're tying & the hook you're using. So the size of the materials in the fly must change according to the size you're tying. You must keep the proportions to the proper ratio when making the same pattern in different sizes.
Ultimately, the proportion of your fly is only a generalization - there are no hard rules you absolutely must follow. If you tie your flies with consideration to the living insect, how "balanced" your flies are, & how pleasing they are to the fish (if that's possible to calculate), your flies will be successful "bug puppets" that will catch fish.