exhaust reducing size?

[stu768]

have a mk2 ph1 and i bought an ashley backbox the pipe work leads back to the down pipe as it should etc but the bore it too big for my downpipe, does anyone no what size my downpipe is so i can get a reducer to size for it? its a 1.2 8v

 

[stu768]

anyone able to measure the downpipe or no how to find out?