GOOD NEWS. . . . The PICTURE UPLOAD FUNCTION IS WORKING !!! The tech folks at the parent company have been able to remedy the issue. You can upload pictures once again. Thank you for your patience.
13-1/4X17 or 13X19 should be pretty close..
If that's the tri-hull Glastron, which I'd guess it is by vintage, the 17 should perform real well.
Do you have a tachometer?