C800 Battery Recommendation

Hello! My C800 MY2007 uses YTX-12-BS 12V 10Ah as a stock battery. I'd like to replace it with a different one with as many amp hours as possible while still having it located under the seat like a default one.

Do you have any experience of yours which battery is the biggest one (by big I mean Ah) that physically fits into an under-the-seat battery compartment of the C800 please?