The standard plane for both airlines is the Cessna 208 Caravan. Both airlines have some smaller planes (typically the Cessna 206 Turbo) and Maya also have a few Britten-Norman Islanders. The Islanders are twin piston-engined, the others are all single and mostly piston-engined apart from the Caravans. Maya do sometimes lease a larger aircraft and currently use those for their Cancun-Belize service only. Whichever airline you choose it is most likely you would fly in a Caravan. I believe I did hear of a new service south from Belize, but I don't know where to, what equipment they use, or even if it's operating.
The Caravan is indeed a small aircraft, but as small aircraft go it's pretty big! It also has extraordinary carrying capacity and an excellent safety record, even though it's a single. Not sure of the seating capacity, but it's maybe around 15-18.