They probably work sealed, I haven't used a sealed box in over a decade, they are a waste imo.
For ported I would go as small as 1.8 and as large as 2.3 depending on what your goals are. I've tuned them at 31 and 35hz and I liked the lower tune better because they have a fs in the mid 20s and can extend low with ease.
Great subs, probably some of the best entry level subs ever made. Very efficient and clean sounding, they have that warm and tight Boston Acoustics sound that I love.
These are my ancient vids with G1s and G5s in the same box. About 4.1-4.2cf tuned to 31hz. G1s were 2 ohm load G5s were 1 ohm
G1s:
G5s:
Those same G1s in that box did a hair under 141db in a friend's Mazdapseed6 on a MTX 1500. The subs really hated that level of power lol. They were louder than his Diamond D6 12s by .5db, the D6s were in a box supposedly built to Diamond specs (small and high tuned)
At peak SPL there was about a 4db difference between the G1s and G5s and that was earlier in my testing days so I think there was definitely more room to go up with both models.