I think it is called a foot gauge. Don't know if you would be able to buy one though. I hate Clarks for measuring feet.
I used to have a Saturday job in a shoe shop and we used to put talc in chidrens shoes to see their toe outline (especially good for those hard types of shoes when you can't push down and feel where there toes are).
A foot guage or measuring gauge. I think that startrite do a measuring at home service.
To be honest the gauge is just the starting point - not every shoe will measure the same way as it depends on the individual last. It is probably better to find a shop where you trust the staff (and go midweek, never at a weekend).