I've seen citrus plants with tiny fruit in pots as small as a gallon at the box store. Obviously, the larger the pot, the more fruit and the larger the fruit the plant can mature. Personally, I use 3 gallon to be the minimum size to expect fruit harvest. And that is an intermediate to the 5 and 7 gal that are my max size (due to winter move sizing). I have a lemon in a 5 gallon pot, that I keep trimmed under 7 feet to bring in for winter. It has carried a dozen lemons at any given time.

Pot size isn't really a factor, citrus blooms when it reaches a high enough node count. For a seed grown tree this can take 5+ years. For a cuttings or grafted tree it takes the node count of the mother and can fruit in a half gallon container.

In my experience it all has to do with the size of the root ball. You don't want to go too much larger or you invite moisture issues, smaller and the roots can bind and girdle since they won't have anywhere to go. I usually just go one or two pot sizes up from the current pot when the tree is in need of a repot. It all depends on how fast the particular tree's roots grow.

My New Zealand lemonade tree in a 6 gallon pot produced 7 lemons and my seed grown Meiwa kumquat produced 32 fruits with a 4 gallon pot

Meiwa tree with 15 visible ripe fruits

New Zealand lemonade tree with 11 visible of 2 lemons left near ripe.

Steve

You don't need a huge pot to grow fruit (as others have mentioned, 5-7 gallon is sufficient for decent harvest). But obviously your tree's roots will continue to grow with age so eventually you will need to go bigger or root prune. I've seen some amazing pictures on this forum from others who are really good with roots. You can work the root system to become very efficient. But that is a skill I don't yet possess.

Ken B, what do you mean a grafted tree takes the node count of the mother?