a minute amount of anti-matter that comes into contact with matter would nullify each other in a chain reaction and release a vast amount of energy. the amount of energy released would depend on the amount of anti-matter that comes in contact with matter.

red matter only becomes a black hole when it comes in contact with nuclear matter. so the size of the black hole would depends on how much contact occurred. so no matter if it's a drop or a whole ship of red matter, if there is only so much of nuclear matter that it could react with, it would still create the same size of blackhole.

what happens to the rest of the red matter? well, in theory, the center of the blackhole is a singularity. we dont know what happens to matter in a singularity. one possibility in this story is that it gets transported back in time, although in reality it's extremely unlikely that matter would retain its properties in a singularity.

i recommend doing a bit of reading about blackholes to understand its properties. dying stars arent the only thing that could create blackholes. it's all quantum mechanics.

i have no problems in suspending my disbelief over red matter. if u really wanna talk red matter, then i suggest starting with decalithium ores that power the ships. then venture into how the impulse engines work using powers derived from processed dilithium (not decalithium, mind u ). then go over the transporter and phasers and proton torpedoes.

there are lots of other things u should start questioning before u should even question about red matter, bcoz even to me red matter is quite bizarre to explain. it's the first time ever it has been used in science fiction.

by the way, tricorder is the gadget that looks like a flip-phone. i'm not sure if it's being used in this movie, but it was in earlier movies.