I have a Miami, FL PO Box address but am currently living abroad. What do I put as my state of residence address?

The answer is "Your state of residence is where you consider your permanent home to be."

Generally, you're a resident of a state (or country) if you intend to either stay there permanently, or  return there after a temporary absence. It's where home is – where you come back to after being away on vacation, business trip, overseas or out-of-state employment, or school. Many factors are considered, not the least of which are where you are registered to vote, own homestead property and are licensed to drive. 

In the Personal interview, you are asked to list your "State of Residence". One of the options in the drop-down menu is "Foreign or U.S. Possession". 

If you have, in fact, abandoned your previous state and established residence in a foreign country, you will select "Foreign or U.S. Possession" and provide the date you established residency in the foreign country. 

Date You Established Residency 

The date you choose as the first day of residency can coincide with one of the following events: 

You or your spouse arrived in the state or country 

Your belongings arrived 

You or your spouse started work 

You started renting your new place 

You purchased your new home 

You or a family member enrolled in school 

You or your spouse registered to vote 

You or your spouse applied for a state driver's license