as above had to put a new battery on my car, the old one was the original 1999 one, used the car fine yesterday no indication that the battery was dying, went to start her today......dead well not quite prehaps life support

anyway my son works for pug garage as van driver and got me one cheap, this one he assures me is right however it is prehaps 2" shorter in length than my old one ? anyway car starts fine now windows work etc....
is there anything else i should be aware of or need to watch ?
it took about 3 trys to start it up as if no petrol then it fired up and idled ok.