Someone else will be along to discuss engine and transmission, I'm not going to do that.
Not sure anyone can tell you what length bus is right for you and your family. Maybe one way to help you decide is to find a space big enough and lay out the size on the floor with tape and start 'placing' your furniture in it. That'll give you a good idea of what size bus you could get by with. Or since you said you had some kind of trailer you could just base your estimations on the size of that trailer and how well it worked for you.
My bus started out as a 35' dog house, 11 window. I chopped off 3 windows worth of the rear so I have an 8 window now. That gives me 21' behind the driver's seat and my bus is 90" wide inside (I just calculate it as 8' wide cause I'm lazy) so I have about 160 square feet empty. I'm going to do a queen size platform bed so I'll have about 40 square feet of storage under that as well. This just to give you an idea of size in my particular 8 window bus.
The thing I noticed as soon as I read your post is you're thinking of converting a bus to take a 2 or 3 month trip. It generally takes many months to years to do a conversion depending on whether you go bare bones or stick build a house in it. You'll also have to spend time going over (or having it gone over) mechanically which is a step you SHOULD NOT SKIP. Lots of folks do elaborate builds and take their first trip and end up on the side of the road because the bus wasn't mechanically sound - bad JuJu.
At any rate, welcome to the forum. I'm not trying to be overly negative but you need to be thinking clearly about the job you'd be taking on. If you're thinking about doing this for one 2 or 3 month trip, I suggest that you consider finding a used RV. On the other hand if this is going to be your 'forever' adventure making machine then understand the time investment and go for it. This place will be here to help you with all your insurmountable problems.