FOM: unsurveyability

[Kanovei]

Holmes: 
 
<For example,
<if one is an ultrafinitist, one may flatly deny that proofs of
<sufficiently large finite size whose existence can be deduced formally
<(though they are in practice completely unsurveyable) even exist, much
<less prove anything.  

Is there any example of a proof (concrete proof, I mean, 
not a hypothesis of its existence), seriously denied 
exactly by the reason that it is too long ? 

V.Kanovei