Touchscreens usually have in their report descriptors information about the physical size of the touchscreen they're attached to. For example, it would be possible to see that a touchscreen was 23" big, so gnome-settings-daemon could automatically match it with the 23" display that's configured. That information is currently not exported to X clients, so gnome-settings-daemon cannot consume it.
the XI valuator class structs have the range and the resolution in units/m, that should give you the physical size (where the device doesn't lie about it). Is this good enough? http://cgit.freedesktop.org/xorg/proto/inputproto/tree/specs/XI2proto.txt#n985
Benjamin Tissoires pointed this out, and yes, it's good enough for us to use even if the way that it's exported is slightly more convoluted than I would have expected.
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