need to scale / edit something but no clue on how to 1:1

following issue, i have a CD keychain, ( CD and Case like PS1 case) and i want to design a cover for the CD and for the case on gimp, now the issue, i dont know how big i have to do something to make it 1:1 like i only have to cut it out of the paper and then just stick it on the stuff, anyone know a way how to do that?