Small, meaning what?
Small in size?..........most roof units are about the same footprint and external dimensions, except the Carrier standard model (which has a slightly smaller footprint but is much taller), which is getting very hard to find and Carrier no longer supports the RV units that they used
Height? All three companies (Carrier, Dometic-Duotherm and RVP-Coleman make very nice low profile units.
Capacity?........RV roof units start at about 8000 BTU (Coleman Cub) and go up to 15,000 BTU. You can get them in A/C only, A/C with heat strip or full A/C - heat pump combinations
Reliability? From my 10 years doing RV service work, they are all about the same. You can purchase extended warranties that are direct from the respective manufacturers like RVP (Coleman) or Dometic (Duotherm) that will give you an extra 3 years on top of the original 3 years for a nominal fee of under $100. I usually suggest this simply because I have had clients that have used it in the 4th-6th years of ownership and the companies have ALWAYS honored them, saving the clients as much as $1200 plus installation for a new unit.
That's my answers and I'm sticking to them.....
Let me know if you have any other questions or need any other help.