Get the biggest tank you can afford and have room for.
You and the fish will be happier that way. If you are going to put fish in with a Betta, I wouldn't get anything less than a 10g.
I kept 6 Harlequin rasboras and a male Betta in a 10g for awhile and they lived happily together, until the rasboras got an upgrade.
You'll have more fish choices with a 30g. If you are putting the Betta in, I'd make sure your filter isn't too strong because that can stress them out.
For fish choices, I suggest 8-10 Harlequin rasboras, 6-8 Cories, the Betta, and maybe a few Otos.