You need to answer these questions first:

(1) Do you cook in your pot or use the Freezer Bag "Cooking" method (boiling water and pouring it into the freezer bag containing your dried food, allowing it to rehydrate for 15-20 minutes in a cozy)? (See the link to Sarbar's website for details.)

(2) Do you drink lots of hot beverage or just one cup at a meal?

If you only heat water to a boil and pour it over your dried food, saving the rest of the hot water for your beverage, figure out how much water you need per dinner and add how many cups of water you want for your beverage. You of course want an inch or so on top to allow for expansion and bubbling. I use this method (I hate washing dishes!). For solo use (drinking only one cup of tea) I get along fine with a 550 ml. pot. I have a 1300 ml pot for going out with my 3 grandkids, but I take more fuel so I can boil water twice--once for rehydrating our food and once for our beverage (hot cocoa, in the case of the kids). As Sarbar says, I'd need less fuel if I took a bigger pot and boiled only once.

If you're cooking food in your pot, you need more than double the pot capacity you'd use for heating water, so your food won't boil over. If I were doing actual cooking, I'd use my 1300 ml pot for solo use and at least double that capacity for two persons. For beverages, you have to pour off the boiling water before adding food to the pot and drink your beverage while the food is cooking, or take a separate pot just for heating beverage water. Some people scrape out the dirty pot and use it to heat their beverage water, but I personally am not fond of rice or macaroni bits in my tea. Your Mileage May Vary!

Some like to rehydrate their dried food in their cooking pot using a cozy that fits the pot. Here, as with actual cooking, you are faced with the choice of drinking your beverages before dinner, heating beverage water in a foody pot, or carrying a second pot for beverage water.