You will have to determine how much space you need. Many folks feel ey need a 3000 sq ft house or bigger for 2 or 3 people. We are comfortable in 1600 sq ft for the 3 of us and use to be 4. There is a couple on here that full time in a 24' class c. They have a video of the inside of their set up. Looks comfy to me.
Another thing you will want to consider (and it may not be an issue for you), is your health. Your health may help to determine what you full time in. If you get a towable (TT or 5th wheel), it may require being more physical during set up and take down. A motor home with power jacks, will make it easier on you. Also, you might want to consider the height of the rig. A 5th wheel or class a (and many tts) will sit up higher than something like a class c or class b. trust me, going up and down those stairs can get tiresome, even for us mid 40 years old. We are picking up our new class c today. We are trading in our TT with bunkhouse as it was an overkill for us. We don't need the bunkhouse with just one kid. Plus we learned some things over the last few years of what we want. The class c made more sense for us. We would of gone class b if it weren't for our daughter, but we do have another 7 yrs or so of her at home. My in laws had the big $600,000 42' DP's before. They are nice, but I don't think they are for me and my budget. Lol
Just remember, the only restriction you will have about the size of the rig is that the bigger it is, the harder it will be to find a place to park it. Or rather, your choices will be more limited.