How to get an accurate estimate of the Sq.ft?

It depends if the building is a co-op or a condo.

If it's a condo, the square footage will indeed be in the offering plan, which your lawyer should be able to get from the managing agent. Be aware that the square footage will be overstated from interior square footage because:
If it's a co-op, it's a little trickier. Co-ops don't have exact figure assosiated with each unit. Agents have been sued over stating co-op square footage, so often we don't or put an approximate figure. You can hire an appraiser/engeneer for a few hundred dollars and he will mesure it. You can also mesure it yourself while viewing the property.

If it's a townhouse, Property Shark will provide you with building dimensions, and that helps some too. If you know a building's footprint is 25 feet by 63 feet, you know you're not going to see 2000 square feet of apartments per floor.