Changing the collision box size for tokens with sight?

Is this possible? and if not, why not? Can this be a feature? It would be helpful in auto-calculating whether or not a token can fit into a small space or not.

It is not currently possible.  Depending on your use case, you could take your token to an external art tool and adjust the space around the graphic to get a different relative bounding box, but you'd forever need to keep the token at an irregular scale on the tabletop.  The existing blinding box is pretty good for most tokens, what case are you hitting that is causing an issue? You can create a suggestion in the Suggestions and Ideas forum for this. 

Well, like making a vent or entrance that your character must be a specific size inorder to fid through without me just going "Oh yeah you fit" or "No you don't"