Sq. Footage from listing and appraisal are widely different - what to do?

Check the fine print in the sales listing and the contract. They usually says that it's an approximation. Some realtors include unlivable space (i.e. inside the walls) when calculating square footage so they'll tell you that you should've gotten an inspection prior to signing the contract.

sq ft quoted may include your share of common areas ?

What did your lawyer say?

Was this enough to affect the bank's willingness to provide sufficient financing? If not, does the contract contain a financing contingency?

I assume you aren't talking about new condo construction where I think sq. footage is spelled out in the prospectus or offering docs and how it is calculated is explained (I could be wrong--I'm not a new condo expert).
In co-op re-sales, I've never seen sq. footage representations incorporated into a contract of sale. It's a caveat emptor situation. During your due diligence, you were perfectly at liberty to have the place measured, to review the offering prospectus listing of sq/ft., to take a calculator to the floorplan measurements and see if it is about right or even to compare the floorplan measurements to the actual measurements of the rooms using a standard tape measure. The result would have been the basis for negotiations before you went to contract if you found discrepancies. In addition, the price of the unit is often related to comps in the building and if all units in a similar line that are comps were in keeping with what you are to pay, -/+ a few sq/ft really doesn't matter. I'm not sure I understand how you feel the discrepency now figures into the sale if you are already in contract. Are you saying the price per sq. ft. is more than you thought and therefore the negotiated sale price should be adjusted? That isn't how the terms of a standard Blumberg contract of sale work. Again, I think this would have been a due diligence issue and that train already left the station.

Should have been: "If SO, does the contract contain a financing contingency?"

Thanks for the comments so far. I have a financing contingency, and the bank has not given a final commitment.

The strange thing is that the comps in the appraisal show other apts. in the same building, but some of those are 10% HIGHER appraised vs. listed sq. footage, based on the floorplans. In other words, smaller apts are being appraised with more sq. footage than this one--so it's not purely a measurement issue. (ie. it's not like every unit is being measured to be 10% smaller)

This happenned to my wife and I too. We're at the same stage you are, but we've been in contract for 8 months and the original appraisal was dead on. However, we had to ride a new one and this one is 100 sq ft off and obviously affected the appraised value. Our mortgage broker is swearing that he's going to 'fix it'.