Accordion sizes.

After deciding the type, you will need to decide the size of the instrument. Many keys means harder will be to play, so if you are a beginner you might consider a smaller instrument.

For example if you choose an accordion with Stradella bass system (see the picture on the left showing a 72 Stradella bass system ) like a piano accordion or a chromatic one, for beginners a 24 or 48 bass keys are recommended. Also more keys means more reeds, more reeds means more heavy, more heavy means less portable.
Many players have to compromise between portability and range of notes. 48 and 72 bass are the best options for most folk musicians, but you only get the full range of notes, and variety of sound from the registers on a full size 120 bass.
The piano accordion with 120 bass and 41 treble keys is the full size model, the bass side offers a chromatic scale and all the chords, arranged in 6 columns of 20 buttons. The six columns are: counter bass notes, root notes, major chords, minor chords, 7th chords, diminished chords. An 80 bass has 5 columns of 16 (omitting the diminished row), and a 37 treble note keyboard. A 72 bass has a 34 treble note keyboard, but retains the 6 columns bass. You can click on the left picture to see a close up for better understanding.
So as you can see there are many accordion sizes that you can choose from. The best learning path is to start with a small one and learn your way through till the full size one although it might not be the cheapest solution.