Low Appraisal - County SF vs. Appraiser SF Question

[Matt Sauls]

I just received the appraisal back on an investment property I'm purchasing that came in $5k low.  My purchase price is $140k and it appraised at $135k.  

As I'm scouring the details of the appraisal I'm noticing differences between the appraiser's listed SF for the subject property as well as several of the comps used when compared to the county tax assessor's website.  These differences range from 50sf up to 120sf in some cases.  

When factoring in the adjustment $/sf he is using, these differences in SF would raise the value almost up to my purchase price of my own property and some of the comps.

Do I have a leg to stand on to appeal?  Or, does the appraiser's field measure trump the county website?  What is the most successful way to appeal for those who've had success?

If not able to adjust with the appraiser, does anyone recommend going back to the seller to renegotiate based upon the difference in SF from the listing and what the appraiser field measured?