Of course the size and weight of an RV trailer is limited to the capacity of your tow vehicle.
As far as the size of a RV trailer you would be comfortable in while on the road is a matter of personal needs and comfort. For ten years now, we have been traveling for 6 to 8 weeks at a time with a 24' TT and found it enjoyable (my wife and I and our Golden). Before retiring we tented for years, so for us, our TT became a luxury cabin.
Some couples would find that small of a TT intolerable for that length of time, still others make do with less.