There isn't a size restriction on normal suitcases/hold luggage, only weight. The size thing tends to refer to oversize sports equipment, diving gear/golf bags/skis/surf boards etc. But then there would be excess weight to pay for as well. 15kg isn't much, when you take into account shoes, toiletries, beach towels etc as well as clothes. DH's case measures 62x41x28cm, and when full can go over 15kg. I guess it depends whether you are prepared/able to do lots of laundry on hols, and take less clothes in the first place. If your two cases weigh over the 15kg, you will be charged for the excess weight, but if you take your three cases you're entitled to, you will be able to take more stuff, free, as long as each weighs under 15kg. The buggy will still be free, but do check with the airline.
I have invested in a luggage weighing scale, a hand-held suspension type, which is really helpful. Thomsons used to allow 20kg of free checked luggage, but have now pulled into line with all the rest, so we have had to pay extra for our next holiday's luggage entitlement.
Be careful with your hand luggage, though, as sometimes they get weighed at check-in, and each item must be less than 5kg and able to fit in the overhead lockers. Interestingly, this doesn't seem to apply on Easyjet, especially if you have no checked in luggage for the hold, something they are keen to restrict, because loading/unloading hold luggage limits their turn around time. Tip: buy magazines after security as they tend to be heavy.