On a through ticket: no problem. On separate tickets: great risk of misconnecting. Any delay on the transatlantic leg or even a long line at immigration would endanger your connection. The risk of the hand luggage being to large for Aegean is the least of your problems. If you reach the gate in time and the carry-on is deemed to big, you will just have to gate check it.
I would not risk this connection on separate tickets, especially to a destination with only one flight on most days (and if you are on one of the two-flight-days, go for the later flight at 14.20!)
In the end my connection was no problem at all.
The United flight arrived early, the connection by train between the terminals was very rapid, and with our EU passports we could use the automatic gates which had no lines. (Even for non-EU passports the line was very short.) Also, no additional security to go through.