Anyone help me pick the best battery replacement

[remanoku]

Hey guys so looks like my battery has been dying recently so guess it’s time to swap it out, the pervious owner has put a smaller then standard battery in.
I’ve been using Tanya.com to choose a battery and that too recommends a shorter battery seen a few posts saying bmw changed over to a shorter one but I’m not sure what to go for, can anyone give me some guidance what are the benefits of a longer battery? Will it last longer or have more amp hours