Okay a few points here to consider:
1) Who represented the 620? the offering plan - the prospectus? or the broker? In the end, BEFORE you go to contract your attorney should have gone over the paperwork with you and you should have seen the actual sq. footage.

2) If you are NOT in contract, adjust your price. If you are in cotract you can break the deal or renegotiate- again TALK TO YOUR ATTORNEY. The agent may have had the same info you had, so you never know. The offering plan/prospectus, etc has the actual sq. footage recorded and that is what you go by, especially for condo/co-op's.

3) What did you Measure? Did you measure the foyer? the walkway to the bedrooms? other rooms? Sqaure footage is a total sum of liveable square footage - NOT just the bedroom/LR. Typically a bathroom is NOT included in sq footage but they may be, as are kitchens. Some places include the balcony in their sq. footage - not my kind of idea of accurate - but nevertheless - see the offering place/prospectus.

Finally - you saw the place right? You thought it was worth $455, and offered as much for it, if you don't want it now b/c of this - then move on. In Boerum hill, $800+ sq ft is normal - so see what else is out there, renegotiate and then reassess if you really want it. good luck :)

I would explain to the seller that as they have it listed it is listed for $733.87 per sq ft, based on the fact that the square footage is wrong you could argue that you would be willing to pay $733.87 per sq ft but for the true 475 sq ft.

You should definitely ask for a price reduction. Quoting the wrong square footage can be actually be terms for breaking the contract. In some cases, it has actually brought on lawsuits for false advertising.

Typically the misrepresentation of the Square Footage of a house can be considered a serious violation depending on the particular board the Realtor is a member of, was the property SQ Ft listed as estimated? If so why didn't your Realtor know the actual Sq Ft? These are some of the questions you should be asking