Keeping a fuel canister in your cook pot is not necessarily the best idea since fuel canisters sit on the ground and you eat out of your pot. Also fuel canisters tend to rust around the base which can stain the pot.

REI Stores will generally have full canisters on the shelves although there have been shortages this year.

As to size the ~100g canisters are ~3.5 inches dia. will fit in all Toaks D95mm (3.75inch) series pots and the ~230g canisters are ~4.25 inches dia. and will fit in all Toaks D115mm (4.5inch) series pots.

The JetBoil Flash takes a 100g and the MiniMo takes a 230g

Most small pots are sized that way.

Generally 100g are better for solo use if you just heat water and a canister can last a week or more depending...The canisters are light enough that carrying a partial and a full one is not an undue burden if you want to used all the fuel and are prepared to pack out the empty...but sometimes only 230g are available so you have to go with those. 230g is probably better for two person use or if you actually "cook" on trail.

A 550ml pot is just big enough if you are only boiling water for one. I like the Toaks 650ml light for that since it provides a bit extra capacity and is slightly more weight efficient. If you are going to cook pasta type meals then a ~1L pot is a better choice. The Toaks 900ml 115mm is a popular choice but there are many alternatives.