nymph size

With flies, in the film, and with dries, I feel confident saying that "a
little smaller is better than a little too big" in reference for patterns
fished to selective situation fish.

My question is this. If you are fishing a nymph deep to imitate a specific
insect, at a time that insect is active and emerging ... i.e. a "selective
nymphing" situation .... and your box is empty in the correct size ....
would you put on bigger or smaller? I don't have a lot of confidence
in my nymphing, though I usually catch some fish when I resort to it.
Your, in depth, experience can help me make decisions with more faith, and,
right now, decide which end of the size spectrum to emphasize as I stock
boxes for the season. Thanks.