Think the correct size is a 096 battery (278x175x190) for the GTI. The Varta Silver line (E44) 77Ah battery is a popular replacement for the GTI and has more cranking power than stock, not that badly priced too for a top tier battery.
I personally have a Hankook UMF57800 (78Ah) in mine as it's a bit cheaper than the Varta and generally works fine in my GTI with all the added gizmos so far.