Peppers in pots - how big a pot?

That sounds like plenty. We normally confine chillis to 2 litre (or 6 inch across the top) pots.

that sounds fine for chillis but I have my sweet peppers in bigger pots than that, the size you grow tomatoes in.

I think its more reliable to describe in litre (volume) size rather than top diameter. The pot you are describing might be a different shape even if it does have a certain top diameter, and volume is a constant or a finite measurement.

the smaller size supermarket flower buckets hold about 7Lt.
That's what I grow my peppers in.
99p for 10 at most stores

Morrison s flower buckets come in 10litre and 5litre. I have my toms  and melons growing in the 10l and pepper Chilli & sweet in the fives, both seem happy enough.

they're sometimes stacked up by the cut flower display, just ask.....most supermarkets sell 10 for 99p, my local coop gives them away