What size flange for stock piping

[97gst_spyder]

I have a Greddy RS bov, and stock piping. What size flange do I need? Heres the site I'M getting it from: SBV Blow Off Valve Type - OVAL ADAPTER | Custom car parts, performance auto parts and truck accessories . They have sizes from 19mm to 38mm.

 

[wicketywacker96]

Try this.
This will take the guess work out of it.