You need to weigh yourself with the sort of clothes you will bock in and then add 10kg that will be the corect weight for you if you are of average fitness
If you are into trampoline or some other similar athletic sport in a big way ( like really fit ) you might need to go 20kg over but if you arn't fit enough the extra weight will slow your progress
so if you weigh 12 stone = about 76kg add say 1 to 2 kg for clothes and shoes =78 +10 =88
I would say you need the 90's or you'll be bottoming them out straight away but do weigh yourself on a good set of scales as my old ones were 7kg out and it can make a lot of difference
Jason