It's not impossible, just a lot more challenging, and pricy. The difference in cost between a small heater and a small-medium is almost non-existant, and so peopel often get the larger heater, which is bad. A quality small tank heater is critical, especially with lit tanks, which oiften have incandescent bulbs that put out a lot of heat.
I'd probably go with a slightly larger setup that's more easily managed. The cost will be about the same, and the betta will be better off.