I have seen on eBay many sellers advertising full size accordions that are 120 basses and 41 piano keys regardless of the treble keyboard size, they are partially rigth, a full size accordion does have 120 basses and 41 treble keys but to make it a really full size accordion it should have also 19" inches of treble keyboard, this means that all together the white piano keys should meassure 19 inches ( accordion frame not included ) so each individual key meassures aproximately 3/4 of an inch which is smaller than any standar piano key.

Full size accordions are very suitable for players with big hands, or acostumed to play standar pianos, smaller keyboards are suitable for kids and ladies or players with smaller hands.

There are many accordions with 120 basses and 41 treble keys with several different keyboard sizes as follows: 14 1/2", 14 3/4", 14 7/8", 15", 15 3/8", 15 1/2", 15 3/4", 16", 16 1/2", 16 3/4", 17", 17 1/8", 17 3/8" 17 1/2" 17 3/4", 18", 18 1/8", 18 1/2", 18 3/4" and 19" etc.  I personally have two accordions with 17" keyboards, 120 basses and 41 treble keys, often called "ladies size, also in the early and late 40's and 50's there were 20" inches keyboars with 41 keys I have a Hohner made in the 40's with such a keyboard, so 19" piano keyboards started becoming as  standar "full size" in the early 50's.

"Full size accordion" term applies only to piano accordions, button accordions are a completely different chapter.

There is not way to tell by looking at the pic if an accordion is full size or not the best way to find out is asking the seller to provide the keyboard size before bidding on it.

Hope this is clear and helpful.

Greetings, Carlos.