Look for plant height and width on a nursery or garden site, regarding pot size, or ask people here about specific varieties. Jalapenos generally grow about 18-24" tall, and aren't going to get very large their first year, so a gallon pot is ok for them. Also, they tend to have small root balls, when compared to other varieties. How do you know the root ball size? Ask, or at the end of the season, pull them out of their pots, wash the dirt off, and look at how much roots you have. (I always repot mine, and prune both the stems and roots.) Cayennes can get much taller, and so I tend to go with a larger pot for them.
Chinenses are all over the board, height-wise, but generally have much larger root balls than annuums, so they are good candidates for your largest pots, as a general rule of thumb. Even though my douglahs are in good-sized pots, I find they are FULL of roots by season end. Other chinenses have larger root balls than the annuums, but not quite as large as the douglahs. The big key here is to watch what's happening, so the plant doesn't become root bound. It will slow production if it does. How do you know if it's becoming root bound? Easiest thing to do is pick the pot up and look at the drain holes. If you aren't seeing roots starting to come out of them, you're probably ok keeping the plant in that size of pot, at least for the time being.
Chileplants.com lists the moruga scorpion as getting 24-30 inches tall, on average. That's not very big, so they'd likely be fine in a 3-gallon pot the first year.
