PTC will say that the min marked width is something huge like 100mm. However in practice this can be a lot smaller. We are shipping product with marks at 25mm and 30mm width. However we are not using these for tracking just links to projects and then model tracking for the experiences. If you want to track of the mark then the mark will need to be large enough for the camera to resolve it as you step back to view the experience.
The only limit we have found on the minimum is the ability of your device to focus close to the mark. View wants to see the mark at a certain size in the camera so with a tiny mark and some device you can't focus at this distance and View will not pick the mark up. Older iPads seem to be very bad at this, other than that the 25-30mm marks seem to be very compatible.
Although I have not tried i don't think there would be a max on marker width.