Layout of the unit is probably more important than size within reason. Look at your lifestyle. If you are a gourmet cook, then kitchen storage and workspace might be important. Do you have pets? Then space for a bed or cat box is really important. Is the wife a glamour queen? Then a big two sink bathroom might be a consideration. Is he a big sports fan? Then maybe an outside TV would be nice. Take extended time to sit in the lounge area. Would both of you be able to see the TV with out sitting sideways or turning your neck? Visualize yourself preparing a meal? Think about placement of appliances such as the coffee maker, toaster oven etc. Do you have a place to slice and dice veggies? Be sure to take a close look at underneath storage. How accessible is the storage with the slides out? Those big double sliding trays that pull way out are really nice.
One important tip... Have the sales person pull in all the slides. Can you get the bathroom? What about the frig, microwave and pantry? We've looked at some units that would have made fixing a quick roadside meal challenging.
I've seen some 38' units I would consider for full timing, and I've seen some 42' units I wouldn't consider.
While we are not full timers, we do go out for months at a time. We used many of the above points in selecting our Winnebago 40R.
