it will get around 7" max depending on alot of things, they could be short bodied because they are hybrids, so possibly less, i can see some Sajica looking colours and markings with the Firemouth looking face, as for colours in general, its a 'maybe' thing, all hybrids are very different.
You could have 100 fry and have 1 colourful one out of the whole batch, one of the main things people hate so much about man made hybrids such as blood parrots, king kong parrots, flowehorns and super red texas is the fact that only about 10% of the fry are ever any good, the rest get dumped into rivers etc.
Also it depends on which way round teh parents were....for example, the picture i posted was the offspring of a Male firemouth and a female convict, if its the other way round, they look different, also the picture i posted was of a good specimen, they are usually much duller.
It is not 100% that your fish has got firemouth in it either, but its most likley, it could be something else like a Red Devil/Midas, which has a similar sized snout. (the orange does not always carry through with the fry)