Along with what Supreme said, there's also far too much evidence for Goku not being on Evil Boo's level before this statement. For instance, Goku knew Gohan's potential far surpassed his own, yet he was absolutely astounded by the burst Gohan had part-way through the ritual and was in awe he could get even stronger. That alone shows SS3 Goku isn't even stacking up to a sample of Chadhan's true greatness. Then there's also the stuff with Gotenks to bring the nail further into the coffin.
Also, if size was the context of the statement, then why would Goku say that "we" weren't a match for Boo like that instead of "I'm"? Vegeta couldn't even put up a fight against Fat Boo without killing himself, so implying things would be different if he was bigger (
) wouldn't work.
"Like this" is most likely just referring to their current limits of power in general.