Lay out what you think and then take away at least a third.
I split mine and DH's into the two separate bags and squash the kids between them since DH's clothes are huge and then I don't get much room in the bag, but the kids stuff is small so it takes less space per quantity IYKWIM?
Even though I know DS goes through more than one outfit per day we only get one outfit each. DH and I often have to wear things twice. We just handwash if necessary. Take as many undies as days you are going though. DD always gets 2 extra pairs JIC.
I know it sounds gross but minimise the toiletries. We have a unisex deoderant that we share (bit of perfume/aftershave makes you smell different anyway), same with toothpaste, shower gel, shampoo instead of separate toiletry bags. Don't take a hairdryer unless you know for certain there isn't one where you are going - they're usually pretty easy to borrow. I only allocate 2 pairs of shoes each (that includes the ones on your feet in transit) and for DH one of those is usually thongs (he has boat sized feet - too many shoes weigh a lot and take up much needed space.
Hmmm that's all I can think of for now. We're going away next weekend so if I think of anymore I'll come back....