I made some 1/20th scale spray bottles for a diorama my boss commissioned for trade shows, and I cast the sprayers separately from the bottles. I also made some air tools and HVLP spray guns as well which were about as small and finely detailed.
The sprayers were about .020 or .030 thick and maybe 1/4" long and 1/8th" wide. Set them up on a piece of styrene square stock with some of the other small parts and used Easy-Flo 60 casting resin, which is water thin. Before it set, I stuck a paper clip into the mold cavity and swished it around to dislodge as many air bubbles as I could. Can't do that for long because the resin has about a 3 minute pot life, but it did the trick for the most part. Came out very nice.
You can cast smaller stuff, but the only thing you have to worry about really is whether you want to mix up a bunch of resin that won't be used. I'd say it'd be next to impossible to mix up exactly enough resin to cast something say, the size of a pin head. It's best to have lots of parts to cast as well as the part in question. With the right resin you should be able to cast nearly anything, but it helps to know a few tricks.