Today, I signed an apartment lease, after visiting the apt. The leasing agent told me it had almost 400 sq. ft. After signing the lease I realized it couldn't have over 300 sq.ft. That 25 % difference makes a huge difference in such a small space.
I paid a deposit and application fee today.

Do I have a grace period in which to cancel the lease.

E

If there is any grace period it will be clearly printed on the lease, either at the top of the first page or right above the signature line. Most likely there is none.

Try speaking to the landlord about it. They may have someone else interested in the apartment. Or they may allow you to find someone else to take over your lease.

Do you have any proof that the leasing agent told you the sq footage?