Leisure battery size to double up under passenger seat

Potentially silly question, but I am going to be fitting a leisure battery to the defender on a split charge. It will be for running a Weco fridge and charging phones / running LED lights etc. Could i Get some advice on capacity and physical size - I want to fit it under the passenger seat with the main starter.

Many thanks

You will be limited by the phisical size,so out with the tape measure.