The size shouldn't be an issue for a betta, but I will say that buying a biorb can be a bit of a headache.
You cannot *ever* interchange parts from another type of system, and the filtration systems in a biorb are pretty substandard. Also, the only heaters (I believe) that fit into a biorb are the ones that biorb sells (not cheap, and not the best quality). So you have very very limited options if you ever decide that something isn't quite what you like.
I agree - they do look cool.
But the additional expense that seems to go along with them turned me off. I had considered this same one for my betta at work, but ended up going with a 5 gal mini-bow instead. I was able to choose my own heater, and the filter that came with it (Whisper) is pretty good quality and is working well for me. If it ever quits, I can fit something else in there without issue.
Anyhow - whatever you decide, I'm sure your betta will be very happy.

Just beware that biorb is like Nintendo. You may be able to afford the system, but the games can get pricey, and none of the "good ones" from other systems work on it. Yanno?
