I'm using a Micra battery, I got mine from Kwik Fit for about 20 notes. This has been great for me, and it's a tad smaller than the standard one. I know however that Rob was running a Merc Diesel battery, although he did have his in the boot.
Remember that whatever battery you go for that the terminals fit the smaller size jap connectors in the R. Thats another reason I went for the Micra.