Doesn't matter what size battery you have if you are drawing too much it will still flatten.
Are you using lots of stereo in car parks etc with the engine off, or will it be driving along all the time ?
There are loads of batteries to choose from these days, lots of different types as well, and also a new EN rating that makes comparison difficult with SAE rated batteries.
The best performance wise is AGM/GEL but anywhere from £130- £190, lesser batteries such as lead/calcium, calcium/calcium etc are available starting at around £60 for a decent one.
On sierras I fit the heavy duty diesel battery as it is about the biggest that will go in, unless you are happy to change connectors, as ford flat terminals are seriously on the way out now, even ford have been using round post since 95/96.
tabetha