Are you wanting a Front or Rear fender pack ?? , front packs should be smaller because of the flopping the front fender does with any weight on the end of it (mud,tools ,etc) , if you put a large front pack at the fork end of the fender then it blocks the headlight , so if you do any evening riding then it makes the headlight all but useless , the rear will take a larger pack , but the taller it is the harder it is to get your leg over the fender
What were you planning on carrying in it ?? , the pre-tooled up packs , usually will have tools you will never use and not have tools you would actually want on a trail ride , so sometimes its best just to get a empty large tool pack and fill it yourself with the items that would be best suited to your specific needs , so it will be sized accordingly , as you can strap it down to contain it to the size you need ,then fill it as much as you need with items such as tire irons that double as axle wrench , and a 21" tube which can be used in front or rear if needed , a small cO2 can made for filling tires , tire patch kit , pliers/small vice grips (small vice grips can be used to hold a broken clutch cable to the lever allowing you to ride back to the truck safely), a flat and phillips screwdriver , a roll of electrical tape , zip ties , so if the kit you buy does not have those items , then you will be pulling out items that came with it to fit the items you will actually need .....
Not sure if that answered any of your concerns or gave you more ....