We have am AB RIB 9 ft w/ a 6 hp outnoard, which weighs 55 lbs, and I think the boat is plenty for four people, mostly it is just the two of us, and I think the 8 ft would have done the job, although when we have four, the 9 ft is the right size. If you are going to have four people with a lot of gear and multiple coolers, etc., all the time, go with the 10-11 ft, if not ho smaller. How far is the boat from the dinghy dock? For just 2 the 9 ft is too big.
Another consideration is the size of the dinghy dock where you are, I am at an old-school marina, where inflatables are not that popular. The 9 ft AB is BIG at the dinghy dock, where space is limited, for that reason I wish we had the 8 ft. Although someone arrived last year with a 10-11 Walker Bay and that was even bigger. Point is, consider where you are going to store it.
If you need to move the boat or store it out of the water the 9 ft AB RIB 'light' weighs 105 lbs and we easily can move it ourselves in and out of the water; the non-light can weigh about 150-160 lbs depending on length.
On towing, I would not get an engine heavier than 55 lbs, unless you always tow the RIB with the engine on, which depending on the seastate may or may not be a good idea, at 55 lbs it is not that difficult to manage.