If it helps, the dimensions of the battery box are:
280mm long x 190mm wide x 230mm high
I recommend fitting the biggest & heaviest battery you can squeeze in the box, unless you are trying to save space.
Make sure it delivers 600 cold cranking amps as a minimum measured using the EN standard
, what most people overlook is there are two different cold cranking standards, SAE & EN.
The American SAE standard is measured at a higher ambient temperature, so 600 CCA in SAE is more like 550 CCA in the more punishing EN (English) standard.
Ignore marketing & brand names, the only true indication of battery quality is weight; simply put, the heavier the battery the more lead is inside it.
Choose the heaviest battery that will fit in the box and select one with AGM technology.
AGM batteries are a big improvement over regular wet battery technology, AGM batteries recover better from a discharged state & typically last longer (especially if you suffer a few unexpected full discharges).
This one looks pretty good value:http://www.performancebatteries.co.uk/product/AX+5...
To assess it's quality I would ring them up & ask them how heavy it is.
As a guide an average quality 072 battery will typically weigh 17kg, a decent one will weigh 19kg, and a top of the line Odyssey PC1500 with exactly the same external dimensions weighs a massive 22kg.
Keeping the battery on a decent maintenance charger will greatly extend it's life, if you use the car infrequently & don't have access to mains electricity disconnect the battery.
A simple battery disconnect switch is an effective solution, I use the excellent "Battery Brain Gold" device because it allows me to disconnect the battery using a remote control after locking the car.http://www.batterybrain.co.uk/productspage.php
Even when disconnected all batteries have a self discharge rate, but AGM batteries have a significantly lower self discharge rate than a conventional wet battery.
A disconnected healthy AGM battery will hold enough charge to still start the car after 8-12 months of storage.
However, connecting your battery to a quality maintenance charger is by far the best way of keeping it in peak condition.