2.5mm

My bike came with a multi tools with lots of allen keys and screwdrivers.  I foolishly assumed these covered all the fittings on my bike.

Our riding at the weekend I discovered that there wasn't a 2.5mm allen key on the tool, which turned out to be crucial.  An improvised repair was effected, but as soon as I got home I bought the right tool for the job so if it happens again I'm prepared!