The longer the trailer the fewer spaces it will fit in at public campgrounds and the ones long enough may be already reserved for taken. The shorter the RV the easier it is to wing it and find a space that works.
There is also more difficulty the longer the trailer in backing into spaces or getting into and back out of gas stations. You can compensate for a long trailer by going to a 5th wheel but that means a heavier duty truck.
I would buy a use trailer that will work for the next several years. When and if it comes time to buy a different one you will have a lot more knowledge as to what is most important.