Hi,
The outside diameter is 5.5mm. Neuros use a connector that has spring contacts to grip the center pin which is around 1mm diameter. If you look in various catalogues you should find a 5.5 x 2.1 connector that has a yellow tip, this is shown on:
http://en.wikipedia.org/wiki/DC_plugI guess the actual connector can accomodate a variety of center pin sizes up to about 2mm. In the UK the dc power connectors can be purchased from Maplin, either singly or as a set with a flying lead. Neuros can probably provide a digi-key part number but their outrageous postal charges make them unpopular in the UK!.
If you are using the R2 on the move then get the right angle version. A small piece of tape over the lead will keep it in place.
Cheers
Pat