Floor plan, floorplan, floorplan!
Look for how the floorplan will work for you. Then when you find something you like, look for "gotchas" in the floorplan.
Can you use the kitchen/toilet with the slides in?
Are there any pitfalls in the floorplan like doors that can catch on slideouts coming in, if the door isn't closed?
Are there aspects to the floorplan thet "we can live with that" that will become bugbears after a while?
Then get on the internet and look, look look. Take your time and really consider whether you've picked the right one. If you can, go back to the dealer or another dealer and go and walk through/sit in it for a while.
We spent nine months going through this. We didn't have the option of re-visiting the dealers and viewing the trailer, so I had to do it all, over the internet. The Keystone website has 3D interactive viewers which are VERY useful for viewing floorplans.
Next, work out your towing weights and lengths. Be realistic about this. A 43' fifth wheeler gives you a LOT of room inside, but where ya gonna park it? A lot of the state parks we go to, we see some really big rigs in there, but I wouldn't want to be finding a site for a 43 footer, nor backing it in. As you to up in size, the purchase and maintenance costs go up, also the fuel consumption to move it from A to B. If you want to store it in the off season, where can you accomodate it? A lot of 5ers are very tall and you might have difficulty finding covered storage for it. I would reccommend that when you put it in storage, you have it under cover. The sun is a hard task master on roofs and that is a place of significant vulnerability for RVs and where you can incur big repair bills.
Finally, if this is to be your first rig, I would suggest doing what we did. We rented an RV (Class C in our case) and did some travel in it. When we stopped at parks, I talked to people who had TTs of a similar size to what I was thinking of. Without exception, they invited me to have alook inside their trailers. I forund most people very friendly and proud of their rigs and eager to show them off, and talk about their strengths and weaknesses.