Betta are pretty intelligent and independent fish, so yeah I think removing the dividers is a good move to give it swimming room. Personally I'd not go with less than 20L, preferably 40L. Water quality is easier to maintain, too. They seem to love having a decent sized territory and plenty of stimulation so long as the water isn't too deep and the current isn't too strong (more of an issue for those with encumbering fins - those closer to wild form like plakats can swim well enough). Plenty of objects, plants, and some larger shrimp or other inverts is nice for keeping them stimulated, and regularly rearranging the scape seems to excite them a lot.
Example: My rescue betta had mobility problems due to fin structure issues but he still patrolled his territory, swam over and under everything he could find, and moved light weight stuff around by shoving it. He also enjoyed trolling the cherry shrimp culls by swimming into a mob of them on their food. He didn't like to eat their food or try to eat them - he just liked to watch them scatter if he swam into the group with his fins fluffed out. He'd then sit on the shrimp crack and wait for them to gather around again before flaring his fins to scare them. Can't say all bettas will be this placid to shrimp though!!!