How many are in your family, and what ages for any kiddos? Any fur friends going along? That can make a difference in the floor plan that you're considering and will impact the trailer size.
For example, if you're going to need sleeping for 3 kiddos, you'll probably be looking at a trailer offering 3-4 bunk beds. Where they are located in the trailer can make a difference as to length. Are the bunks lengthwise or crosswise?
Previously I had a Keystone Outback 25RSS that had bunks in front crosswise, and 2 slide outs. Would accommodate 4-6 on board. That, to me was close to ideal for length and floor plan. Never had any problems with any site limitations, either commercial park or NP/USFS CG. Box of the trailer was a bit over 22 ft, total length was about 26 ft. Rear slide was another 5 ft but could overhang the rear limits of most sites.
Find a floor plan and length that fits your needs now and in to the future, unless you specifically know you're likely to buy something different in a few years (ie, kiddos getting older and not likely to be going along a few years down the road).
Something to consider -- RENT a trailer for a few trips out, to see what would work for your family and the size you're interested in. Try out that 30-35 ft trailer in one of your favourite spots, see how it works for you. May find that you could go with a shorter length with the same basic floor plan, or that 30 ft is doable for you.
