It has always been my experience that when it comes to flies that the smaller the better. I too when I first started out thought that a size 18 was microscopic. Now that I've done more research on the matter I think the size 18 is mighty large these days. Especially, after tying midge flies that are size 22 & 24. I think the smallest is the size 32 which is truely microscopic.
Anyway, as to your question are they to small I don't believe they are. You should be able to catch a great deal of trout with the sizes that you bought. Yes, the stores carry larger sizes because most of them are tied over seas in bulk and the larger versions of the files are usually easier to tie.
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Hope this helps.