What is the max length travel trailer you think is practical

Not a TT but a 5er. 27'10" long. Minus the overhang when towing and parking and it becomes 23'10". Perfect length IMO for overall use. Dinette slide out and two recliners in the rear. Two places to sit are a must for us.
It's just common sense. The larger you go the less choices you have and vice versa. You can't have it both ways all the time.
Try googling some of the places you want to visit and see what size sites are available.