lots of room for luggage (smallish opening though) if there are only two of you you'll be fine. If there are more than two the people in the backseat will not be happy if they are old enough to speak.
This and Immortal's reply are the best answers so far. The Mustang coupe's trunk space is surprisingly large for a car of its type, but the opening itself is a little restrictive.
The space in any trunk is VERY irregular; when a manufacturer says it will hold x.y cubic feet, that's talking about filling the space with something like ping-pong balls and has little meaning for pieces of luggage. So use the dimensions that follow as a rough guide only:
The main, rearmost portion of the trunk has a central area about 44" wide (between the wheelwells), 25" front to back, and 17" high. This is where your largest pieces of luggage would have to go.
Forward of this is a smaller, more irregular area about 37" wide, 14" front to back, and 15" to 10" high depending on where it is measured. A standard-sized carry-on would easily fit in here.
Behind the wheelwells, the 44" x 25" x 17" area widens out to about 55" x 14" front to back.
The unobstructed opening at the rear center of the trunk lid measures 25" wide x 15". Just forward of this, the opening widens to about 36" but is only 9" front to back -- see photo below:
Note that the dimensions shown in the photo are for the trunk opening only, and NOT the actual trunk space within. If you can get a piece of luggage through the opening, there will be plenty of room for it inside the trunk. Also, all of these dimensions are with the rear seat backs folded up. If you don't need the back seat for passengers, you can fold the seat backs down and gain a lot more space for your bags.
Hope this helps, and enjoy your trip to Florida!