If just overall length, i.e.,if the 5er is 35' long from front of pin to the farthest point on the back of the 5er, your combo will be shorter because approx. 2' or so of the 5er (ball parking here), will be over the bed of the truck....IF you're taking 35' of living space in the 5er, it's actual length could be 37' or so, ( again,ball parking) , but 2' or so of that would be over the bed of the truck so, it could be same length as the combo you have now, (again.....ball parking).
Usually dimensions on the 5er, inside length and "towing length" can be found on manufacturer's website, or found in brochures.