One thing to keep in mind: the insulation/body and the compressor take so much "fixed" space and weight that doubling the interior capacity from like 23L to 45L does not increase the overall weight or volume of the fridge itself all that much (maybe by like 25 percent in my example). So if you're going to keep it in the rear or somewhere where size is not a big constraint, you get much more bang for your buck going up a size.
Also echoing other people's recommendations to skip the thermoelectric and go compressor. We had a nice thermoelectric for many years that rarely got used because it was 1) too slow to get down to any useful temperature, 2) never got cold enough, and 3) noticeably louder than a compressor, due to the smaller higher speed fan constantly blowing across the thermoelectric element's heatsink.