i dont have a double cab so cant advise on strength of roof lining.
i will upload a picture of mine as soon as i can find one ( 10 mins )
any size is good i have two speakers in side of roof lining and two in the centre, the weakest part and have no sag which is related to them being there, it greatly improves sound quality although make sure you dont position them near your ears or you will end up deff, just get a stanly knife and cut a hole but be careful, very carful not to make the hole visibly too big or else it looks poo :lol:
hope i helped
cheers kieran