It is nice to tie/fish size 10′s than size 16-20′s. I have to admit, I spent a good many years tying/fishing, with some success, patterns in the size 10-12 range, particularly nymphs. The thought of tying on a size 18 anything and fishing it with confidence did not/could not compute.
But, finally, I had a couple encounters with trout, bigger than anything I had ever hooked, on size 18 flies and I started to consider it as an option. I specifically started tying more wets/nymphs/flymphs in 16/18′s and having success. I have yet to venture into the size 20′s with any confidence.
Pay some attention to small hook’s gape size (go bigger), your thread size (8/0 minimum or smaller to 14/0), magnification devices and less material/bulk on the hook. Of course, basic to all this small fly stuff is studying the insect life of the waters you fish. Study the hatches and learn the probably size of a BWO nymph, a Callibaetis nymph, a PMD nymph, a Golden Stone nymph etc.
The smaller Pheasant Tail Nymph (PTN) and the smaller bead head Chironomid (upper left) have been easy to tie and productive small nymph/pupa patterns to fish with. I would suggest simplistic, small patterns as an option and then pay attention to presentation/location and fish with confidence on rivers and lakes.