well, the most importatn question in my eyes is:
will you be knitting in the round on those dpn? or do you just search for comfort (as i do) and the easier ergonomic work on circs?
if it is just to make flat work... the cable length is of minor importance. The stitches need to fit on, that is it.
I have most my needles in 80 cm length and that is perfect for almost all projects.
big blankets / afghans made in one piece might need biggern needles... 100 cm maybe even 120 cm... also scarfs knit lengthwise might need that.
but honestly: 120cm seems extreme to me... that is a lot to handle on your lap and might get in your way.
if you want to work in the round: 80 cm for pullovers works well for adults - sometimes it might get bunchy with stitches if you have thick yarn, but it still works.
for hats in the round... I mostly use dpn (also because you can try on then) but I do have a perfect 60cm needle (5mm thick) that I love for hats untill the decreases start (then my dpn take over)
if you are unsure and do not specifically want to make small projects: 80 cm does the trick most of the time!