Generally as a rule of thumb on the Astra's , Cavailer's and Calibra's 4 stud discs are generally a 256mm disc , 5 stud is usually 284mm or 288mm depending upon year. Early Vectra's in 4 stud may have the 256mm stuff as well.
Easiest way would be to get some known sized ones from on here or Migweb and then you know what size you are getting.
Joff
Thank you very much mate. My friend has an Astra that might be scrapped, so I could take the front discs off him maybe.