The answer to this really depends on how you plan to spend your time in Portugal, and how many people you're traveling with. If you plan on spending most of your time in a major city, it may be best to stick with a compact or mid-sized vehicle. If you're planning on taking long day trips and exploring a variety of locales, then a full-size or luxury car may be a better choice. If experiencing Portugal's beautiful natural landscapes is one of your highest priorities, renting a motorhome, or even a sprinter van rental is an excellent choice.