Does the length of an IPCC Scientific report imply anything about credibility or thoroughness?
Very hard to say in general.   A 600-page report could be 600 pages of garbage.  Obviously a super-short report couldn't be very thorough, or contain very much detail/depth. 

If someone said "I have here a 600-page scientific report: is it any good?"  I would have no way to know.

If you want to make a claim for the credibility of IPCC reports specifically, I would quote the Union of Concerned Scientists:


  The IPCC’s technical reports derive their credibility principally from an extensive, transparent, and iterative peer review process that, as mentioned above, is considered far more exhaustive than that associated with a single peer-reviewed publication in a scientific journal. This is due to the number of reviewers, the breadth of their disciplinary backgrounds and scientific perspectives, and the inclusion of independent “review editors” who certify that all comments have been fairly considered and appropriately resolved by the authors. 


Recursively, you would then have to support the credibility of UCS, which Wikipedia (which attempts to be neutral) calls a "nonprofit science advocacy organization".