I was thinking a few water bottles, eggs, and other things that need to be refrigerated, Dog food is dry. I have never had a fridge or even seen one to see or compare sizes.
Before buying my fridge I also was very unsure about sizes. Up front I ended up with a 50qt which is likely bigger then I will need in 90% of situations but the price jump from a 35-40 qt to 50 was not that significant so I opted on the side of more room. I found it helpful to use some of the regular coolers I and friends had lying around. Without any ice I could get an idea of how big the interior was.
When determining size this was my order of considerations:
1. Price, its a big purchase but hopefully will last many years. Also look at the prices for different sizes sometimes there is not much of a jump between sizes.
2. What can fit in your vehicle/ where are you going to put it. Mine has ended up staying in there full time just turned off when I'm not using it. And the 50 fit perfectly fine where I wanted it(JKU) so the size jump wasn't a problem.
3. Needs, how are you using it. long weekends, or full week/multi week trips. It is my opinion after using a 50qt that for two adults you could do a long weekend with 30-40qt without much difficulty. You might need to buy smaller condiment sizes and rotate in drinks but you would be fine with minimal pre planning. I did a 3 day two night trip solo and literally just chucked in whatever was in the home fridge with no concern for size. Quart of milk, dozen eggs etc. Now that being said if you have a family or could ever see yourself going on extended trips I would suggest a 50qt for two people and maybe bigger if its a family. I might do a week trip once a year so for that situation I wanted more capacity.
The last thing I will say is once you have it you will use it more then you think. I am a grad student with a wacky schedule. Its great being able to get groceries in the morning before class and not have to worry about them sitting all day. Also if you are traveling to family events, tailgates etc its wonderful to have a mobile fridge.