What is needed to ensure there is a noticeable difference in flies that are only a single size apart is using the same length in relation to the hook shank. In other words, use 1.5/1.75/2.0x's the hook shank religiously. If you tie a #4 with things 1.5x hook shank, tie all of the #4 flies you tie with 1.5x's hook shank. Then when you tie #2's or #6's, tie them with 1.5x's the hook shank of that size. Doing this automatically produces varying lengths of flies in each size since each size hook has a different shank length, the materials will be of different length if you religiously apply the 1.5x's hook shank.
However, if you do not religiously tie with a set x's hook shank length, the flies will be all over the place in overall length. If you tie #4's that vary from 1.5 x's to 2.0 x's hook shank, and then tie #6's that are 2.0 x's hook shank, many of the #6's will be as long as the #4's. Likewise, if you then tie #2's 1.5 x's hook shank, many of them will be the same length as the #4's.
But if you tie all three sizes and religiously keep the material 1.5x's the hook shank (or 2.0 x's as well), there will be noticeable differences between one size and another because of the difference in the length of the hook shank of each size hook.