3 weeks post op, how do I know if my chin implant is too big?

Three weeks is still relatively early to assess your final result.
Swelling can persist especially if liposuction was performed at the same time.
Generally 3 months should be sufficient time for you to evaluate your appearance.
Take photos using a non-zoom lens to try an objective assesment of the result. Take different angles, especially profile views
Traditional zoom lenses on cameras can distort the face and make the chin look larger in close up pictures.
Discuss your concerns with your surgeon who can review your photos or concerns
I hope this helps

Generally speaking, it can take 6 to 12 months to completely heal after surgery. Please keep in touch with your surgeon or his/her clinical team to discuss your ongoing concerns.

You would still have significant swelling just 3 weeks after chin implant surgery. To get the best sense of your final result I would wait at least 3 months to allow your chin swelling to settle down fully. The determination of whether it is too big is somewhat subjective. A profile photograph of your face should allow you to assess how well your chin is balanced with the rest of your face. It is important not to use a wide angle setting as this can exaggerate chin size. Having your original surgeon take a standardized postopertive photo would be most illustrative and allow for the best comparison with your preoperative chin size.

There is no one perfect chin projection for a face but rather depends on the other facial features, especially the nose. Even with this in mind, there are some personal preferences based on your own aesthetic. Certainly, if you decide your new chin isn't to your liking it can be revised. This can be done either through trimming of your current implant or replacement with a smaller one all together.