The big size of the machines?
The low demand for bigger sizes?
The maximum paper size?
Actually depending on the country you live there is a chance you can not find a suitable paper, for example more than 72x102cm.
But for example some Heilderberg machines can print up to 121x162 cm https://www.heidelberg.com/global/en/products/press/sheetfed_offset/speedmaster_xl_145___xl162/technical_data___equipment_11/technical_data___equipment_22.jsp
In case you find a printer that big, you need to see the minimum ammount of units they are willing to print.
The bigger the machine, the bigger the number of thousands you need to print, because it cost a lot to have them paused mounting the new plates.