Sexing by finnage on fire mouths, especially at the size it looks in the photo would be difficult IMHO. Fire mouths are tough to sex to begin with because females just as well as males can have the long flowing trailers along with the reds in the throat and stomach and blue spangling. I am guessing male based on the pointed end of the dorsal (which I know is by finnage in the pic at such a small size along with the red in the throat it's starting to show as well. Of course this is a guess as I haven't kept meeki's in years. Look forward to seeing what this bad boy or girl turns out to be.
Size is around 3" I would say. Ill upload more pics of it tomorrow sometime, for some reason I cannot upload or post on here using my phone. Next time I will have laptop is tomorrow.