Duvet for a cotbed- how do you measure?

[shepster]

I apologise for being such a philistine here, but how do you choose a duvet size for your cotbed? I want to buy a duvet for my 12+ month DS for when he naps in his cotbed (140 x 70 cm), but have no idea if I should buy the same size as his bed (ie 140 x 70), smaller, larger...... Ridiculous, I know. Tips?

Just don't.

Get a single duvetCot bed duvets are too small and the DC always wriggles out. Get a single duvet and put it horizontally :)