I can't find what PSA considers a "Standard Sized" card anywhere on their website, and no mention of not taking Exhibits under a standard sized designation.
For an experiment I inputted a Ted Williams Exhibit onto a submission form under a Standard sized submission, and it accepted it.
SGC's site states anything bigger then 3 1/2 x 5 1/2 is oversized, which comfortably fits Exhibit cards into the Standard sized slot. Just submitted a few exhibits with them recently as a matter of fact.
