A 10 gallon would be great. The are some things to remember though.
Make sure the flow is not too strong.
Give him plants (silk or live. No plastic) with broad leaves so he can rest. You can also get a betta hammock for him to sit near the surface.
Give him something to hide in, whether it's something natural or artificial.
I also just wanted to let you know, that bettas with long fins might be pretty, but they have more problems than bettas with shorter fins. The really long tails (like halfmoon, rose, and feather) are very heavy, and those fish are much more likely to start nipping their own fins. As they grow older, they also have more trouble swimming. Delta tail is probably the longest you should go if you want a healthy fish.
Also, avoid double tails. The way they are bred makes it so that their bodies are actually shorter than normal. This means that they are more susceptible to things like dropsy.
wouldn’t have guessed it was so big!!! Also forgot goldfish could be red. Whoops 
long flowy fins kinda looked betta but then I thought maybe flower cause of the shape and fact it didn’t seem to have moved at all 
what did you end up doing with? Removed it or ?
