Thumbnail Settings

Where is the code that determines the thumbnail size of a post image found?

Currently it seems that my thumbnails are generating at about 128 x 85 px and I’d like to edit these settings.

You’ll see here:
https://www.btccaction.co.uk/2006/08/04/snetterton-btcc-will-be-uks-most-exciting-race-day/
that I want to change the size of the red car.

Thanks