How wide do mirrors need to be for a TT?

It's not likely to come up, but mirrors that don't comply with the law (or common sense even) might just attract unwanted attention, such LEO, your insurance co. or a lawyer, in case anything ever went wrong, or someone was having a bad day.

Also, bear in mind, mirrors are an 'equipment' requirement and as such are NOT covered by reciprocity, you must comply with each state you drive in.