I'm gonna make this short and concise, because I wrote a **** essay in the Backroom boards regarding this character that was given a poetic shrug.
In my opinion the main issue with Charizard is not his size (he can take a good deal of punishment) nor his supposed lack of a GTFO move (he actually has one of the more godly and borderline-broken GTFO moves; Rock Smash), but rather his clunkiness in the air as one of the flier characters in the game.
With the possible exception of Pit (I do not have sufficient experience with this character to judge), the fliers of this game all have a fluid, efficient way to aerially outmaneuver and combat both airborne and ground opponents, be it through their number of jumps, walling/shield-pressuring aerials, aerial mobilty, or a combination of these 3 attributes. Charizard lacks all 3; he only has a half-decent third jump, has no real walling aerial anymore (fAir is a shadow of its former self), and with that big gut, his aerial mobility is just plain silly.
This weakness not only affects his onstage play; his recovery and offstage game suffers just as much. As Charizard, you can only go for a far edgeguarding move once before you gotta book it back to the stage lest you go too far and/or low and you can't make it back because...
1) his jumps don't give sufficient vertical height, and...
2) aiming and sweetspotting Fly... yeah, good luck with that. Fly not only has a really small sweetspot, the actual ledgegrab occurs after the apex of his jump as he awkwardly nudges forward and starts falling. Worse, even when you think you nail the sweetspot just right, there's a hurtbox on his wings for a couple frames which more often than not reaches above the ledge; a knowledgeable opponent can and will exploit that easily.
This weakness wouldn't be so bad if Charizard excelled in other areas. Granted, he has a great spacing game, decent combos, and Rock Smash I swear can be its own character, but with B+, that's just "meh". Other characters have a superb spacing game, great bread-and-butter combos, an overall reliable move to fall back on, AND so much more that makes them genuinely viable to win tournaments.
For a flier, Charyzardz is lacking; he needs an extra ooomph to make him a more fierce competitor, and I believe that can be done in giving him a boost in his aerial play. I proposed adding an extra jump, and some other peeps have also given good suggestions, such as increasing his aerial mobility and tweaking Fly. I would love to see all these suggestions be given a chance, but that's ultimately on the Backroom, which unfortunately they shrug off as deeming it "too drastic" of a change.
Shiet, I done wrote a mini-essay.