Descartes, IIRC, somewhere says something about the vagaries of memory influencing our justification for believing in our memory, and thence for believing in proofs involving many steps that we have to mnemonically track as we go along. Not that our justification is entirely negated, but just weakened modulo hyperbolic doubt, say.
Now, with respect to the preface paradox directly, does this paradox indicate that a properly modest mathematician should not completely accept a proof that goes beyond a certain (arguably vague) boundary of length? I've read that some proofs go over 200 pages, for example. Granted, typesetting will be a factor in this kind of situation to the extent that irregular typesetting can inflate a text's size beyond necessity, but I rarely see full academic writers performing that kind of inflation operation (I can remember one and only one dissertation I read where the font was Courier and the spacing was egregious, so what probably could've been a 300-page document ballooned to ~600 pages). But so I doubt that most long mathematical proofs appear to be so long mainly on account of how they were physically jotted down.
But at any rate, then, perhaps more intriguingly, does the viability of long mathematical proofs undermine the preface paradox instead?