How do we know what size of ink Cartridge to get?

you are so much better off getting a printer from staples if theirs a problem and you bought the inexpensive insurance you cant go wrong i have a brother for black and white print outs and a canon mx340 I bought5 for 70 bucks it scans faxes photo copies and prints b and w color gloss and and color flat to get the correct cartridge tell your local staples salesman your make and model you cant go wrong