What is the scarring like from canthopexy and canthoplasty?

The scarring can be very minimal, but it depends on the technique used. You can see one variation by seeing attached link below. Other techniques are available too. The bigger question why you need the canthopexy/canthoplasty and is the procedure appropriate, which is of course is determined by proper evaluation.

It should be a small incision that heals with a minimal scar.  That said how you do will vary based on who actually performs the work and how your body reacts to the surgery.

The scar is on the lateral side of the eye. It typically measures about 1cm and heals very well. there are options to laser and treat the scar also. Baseline skin tone and sebaceous structure have a big impact on scar so that changes from patient to patient. 

For a canthoplasty a small incision is made at the corner of the eye in a natural fold.  The resulting scar is usually minimal.  The bigger question is why are you having it done and what do you hope to achieve with this surgery?  Make sure you are doing this with someone who does a lot of lid work.