The battery specified for the Favorit (and I believe Felicia) is 063 which has 44 amp hours. However, with the battery clamp design and extra space either side in the Favorit I believe it would be possible to fit a longer and possibly taller battery with the same width, terminal orientation and terminal type. For example, a 065 is 3.3cm longer and has 54 amp hours. Or the 075 is 3.3cm longer, 1.5cm taller and has 62 amp hours.

Or there are Calcium batteries available in Halfrauds which claim to be longer lasting, and have higher cold crank starting amps, but appear to have same amp hour ratings.

Asking as more an academic question really as my couple of year old standard battery seems to last at least 2 weeks quite happily without the car being run, even though I've added a few gizmos to help drain the battery.

No, it's not worth spending any money - unless, I suppose, you think you're likely to sit with the engine off and the electrics on for a long time, or are in the habit of forgetting to turn the lights off when you park at night! An 063 battery in good condition will give you ages on the starter with a Favorit, even in the cold. If it takes more than a couple of cranks to start (injection or carburettor model), it's simply a sign that you need to do some maintenance anyway!

I even used to drive a 1700 diesel with an 063 battery in it - that's probably twice the cranking requirement of a Favorit. Still managed OK, and on one occasion when a fuel line died I was able to drive the car a few tens of yards on the starter to get it off the highway.