I have a tank of the same size, and I've struggled finding good big fish to keep that aren't too big or too small. Here's a list - these may be slightly inaccurate and anyone is welcome to correct me:
Pike Characins - 7-11 inches
Snydontis Sp. Catfish - 10-12 inches
Medium to Large Gouramis - 6-10 inches
Bala Shark - 12-14 inches (You may need to move them out at adult size)
Jack Dempsey Cichlids - 12 inches
Chocolate Cichlids - 9 inches?
Severum - 11 inches
Discus - 10 inches
Angel Fish - 10 inches
African Butterfly Fish - 5 inches
Yellowtail Freshwater Barracuda - 9-10 inches?
Some species of Polypterus - 11-14 inches
Archerfish (Brackish) - 6-7 inches
Other small Cichlids - 7-13 inches
African Brown Knifefish - 10 inches
Ghost Knifefish - 11 inches
Ropefish - 3 feet, lazy, can breathe air
Plecos (Some get MASSIVE though) - 8-14 inches, small species only
Silver Dollars - 5-12 inches
Freshwater Moray (brackish) - 16 inches
Small Puffer Species (Must be kept alone) - 13 inches
Other small Cafish species - 9-13 inches
Geophagus Sp. - 8-12 inches
Feel free to add to or correct the list!