For competition shooting, you are limited to a 4 ft. barrel (with distance penalties assesed for longer, and no grace for shorter). For hunting (see previous posts), you want to take advantage of the internal ballistics, so longer is better. Cherokee use at least a 6 ft. barrel (River cane) with 17 in. hardwood splint darts. Indigenous Amazonian hunters use 9 ft to 20 ft. barrels, to shoot game up as high as 100 ft. in the forrest canopy overhead.
Here in the "modern" world, that is cumbersome. Until you can master a longer barrel in practice, I would suggest starting with a 4 ft. 0.50 cal. barrel and get real good at target shooting. (No sense wasting darts, or not getting a humane one-shot clean kill in the field) Then move up to a 0.625 (BigBore) at 4 ft. - repeat. Then go for a 5 ft or 6 ft. - repeat. Once you can consistently shoot (note - with greater than 4 ft. barrels, move back 1 ft for each inch over 48 in. lenght from the 33 ft. mark, so for a 6 ft. barrel, you've added 24 ft. - you'll be target shooting from 57 ft. (about 17.5 meters). Still aiming at a 6 cm bulls-eye. When you can consistenly score 180 points from that distance, you can safely hunt, knowing you'll get a first shot kill every time (well, if it doesn't move! )