Is it possible to lower the chin by 7mm?

Yes, it is possible.

Make sure to consult with a board certified plastic surgeon with a good eye for the best results.

Best of luck !

It is doable, but 7mm is A LOT in a face. Although every face is different and you'd probably benefit from a large descent, we tend to set the limit at 5 mm, altough it is impossible to say without seeing you in person or even in photo.
You should bear in mind that when doing big movements like that, it is necessary to assess soft tissues and airways and also that bigger movements have high risks of vascularization and are less stable.
If you have a very short face, you probably would rather benefit from a bimaxillary surgery + genioplasty, so the downgrafting is balanced throughout the lower thirds and not just concentrated on the jaw.