Cayenne can be pretty vigourous, if you want a lot of chillis then a 10" is sensible and expect to be watering pretty much every day and tomato food one a week.... lots of fruit though.... If you want it as a house plant then I reckon a 6" pot and expect to water morning and evening (feed at half strength twice a week)... They're quite attractive plants in their first year, but a bit like all chillis are rather brutish in the subsequent years...
chrisc