What is the nut size to install the catalytic converter onto the manifold?

Hi, I’m having a hard time trying to figure out what nut size is required to install my catalytic converter onto the manifold on my car listed above GTI, everywhere I try to buy one it doesn’t include hardware other than sleeve and gasket. I even emailed and asked them what nut size is required and each of them told me they were unable to tell me, my car is parted out, so it is my project car. If you can let me know ASAP it would be appreciated. Thank uou