The thing you'll have issues with is being certain that they are true Melanopterus. There are a number of Brycon that look similar (this was covered a while ago so might be worth having a search) so a 'full grown size' is tough to pin down. I'd expect a minimum of about 12" but could be up to 20+" if it's one of the larger varients.
With Brycon in general, I'd recommend not keeping them solo as they are busibodies, hassling tank mates relentlessly. They wont cause any damage usually but can stress out fragile tankmates. In a group, they'll generally squabble amongst themselves, leaving others alone. The draw back to a group is that you will struggle to feed slower bottom dwellers due to their speed and appetite. For that reason, I would not keep them with rays or fragile catfish species, nor geophagus. that said, they've been kept with agressive tank mates with success, as they are very good at keeping themselves out of trouble thanks to their speed.
If you wanted to go for Brycon with other species, I've found Hilarii are MUCH easier going and better to look at but they do get big- Mine is pushing 18" now. Failing that, they're a good 'movement' fish and would work well with something like a Hoplias Sp. where space does not allow a Dorrado, which they are quiet similar too in many ways including certain personallity traits.
HTH