10"- 12"+ the first year if fed well and well cared for. My 3yr old grown from about 4" is pushing 18" now. ( started in a 20 moved to a 75 then a 120... now in a 1k pool.) 24" in captivity is considered a good sized fish, but they can reach larger sizes in the wild.. ( pretty much double ) so imo the potential is def there to reach those sizes we just haven't figure out what we're ( gar owners in general) are doing wrong.
Most florida gar would be happy in a 180 6'x2' footprint for a very long time. I would personally go with a 30" deep tank though if you can. As 24" even for an 18" gar is imo pretty tight since they aren't as flexible as some other fish. Go much wider and you chance them breaking their backs in a spoke/dart... it's a fine line sometimes.. and likely why many don't hit bigger sizes. Problem is when you put a 6" gar in a 180 gallon+ tank they can get up to speed and break their backs ramming into the glass... Put them in too small a tank and stunting is inevitable... along with lack of exercise ect... I suspect this has much to do with captive sizes.
i've had mine for over a year and he is 15-16 inches long(i bought him at 8-9 inches). i've caught some big ones over 24". this one we measured at 25.25 inches and was released(the forest line is also the swamp line).