I would use 5's too.

A lot of people seem to say 5 gallons for peppers. If you are only planting one per pot, I'll agree. If you are wanting to plant more...I'd recommend the matrix found on Love Apple Farms website. They have a very good chart (link below) for many common veggies. I started using their chart this past year for tomatoes and other veggies and the results speak for themselves. 9 foot plus tomatoes at "first fruit set" using Al Tapla's 5-1-1 mix, olla irrigation, and containers sized as per their chart. The difference in quality, disease resistance and productivity is remarkable. Best of luck to you!

Here is a link that might be useful: Love Apple Farms Container Size

I like 4-5 gallons for peppers. It's not unusual for me to get 18-24 bell peppers per plant per growth cycle. I'm thinking that's good, but have no idea how other's yields compare.

Al