Frost free is going to be the fly in the ointment. Marine/RV are rarely self-defrosting. As far as size, take accurate measurements, google is your friend, there are plenty of sources, start with Defender for a good overview. If you're looking at 12V, make very sure you have adequate power supply. Pay particular attention to wire size, as most of the 12V marine/RV fridges are sensitive to low voltage. Wire that is marginally sized can create a voltage drop that may not appear to be significant, but can cause the fridge to behave in unpredictable ways. Depending on the length of the run, you may need to go to #8 AWG conductors (both + & -) to prevent problems. The additional cost for 12VDC/120VAC may not be worthwhile, when do you NOT have 12V available? Pier queens are perhaps the exception, but for a cruising boat, 120V isn't high on the list.
The other side of that coin is to use a 120VAC apartment fridge that you can buy at any of the home centers for 1/4 of the cost of a "marine" fridge, and simply run it off the inverter or power it from an independent inverter dedicated to the fridge. More availability in frost free, efficiency is good, although you'll have overhead from the inverter. There's a lot to be said for that option.
Most everything "marine" will be using the Danfoss compressor, likely BD35. They are proven, and reliable. Power supply is key, they DO NOT tolerate marginal wiring.
