1. "bigger tank is better": Not always.. in some cases it is better to keep them is a smaller tank. Knowing your individual fish is key to choosing tank size. there is no magic one size fits all tank for gars. This is why I primarily do not recommend any.. It is kind of pointless to do so until you know your fish, it's growth rate and it's individual behavior. If you wish to keep gar, and be successful with them, A key piece of advice is to have on hand multiple housing options anywhere from a 1 gallon tank to a 1000 gallon pond and all types in between..
2. "Gars slow behavior":

Slow behavior? since when? these fish are not slow when they do not want to be. Longnosed in particular are capable of moving very quickly to either escape something or to feed. They are built for speed in open waters.. Shortnosed are similar with enlarged Caudal and paired fins to move quickly in open flowing waters. Even the smaller fined and chubby spotted is capable of moving very fast and they do not hesitate to do so.
3. "Giving only enough room to turn stresses them": No not really and in such a case it very much does reduce smashing into walls. Gars really do not stress that easily over minor things. Water quality is more likely to stress them than tank width..
4. "there should be a tank completely dedicated to growing out gars." Absolutely agree with this and the best piece of info so far in this thread..
5. "gars are not as flexible as say an aro": This is not true at all.. Gars are just as flexible as any other fish with this body shape. These fish are very capable of turning in a radius of half their body size at a very fast speed. It is this quick escape C-start that most frequently results in broken vertebrate.