I would like to use A0 size or any large poster size as a print template for the print widget on ArcGIS Experience App Builder. How do I go about doing this? If not possible, then what workarounds have you done that worked for you?
You haven't mentioned if this is Enterprise or AGOL EB. While I haven't done this yet in EB, I suspect the answer is the same as if you were using the JS API (which I have done). In that case, you have to create print templates in ArcGIS Pro and publish them as layout templates.
See the Enterprise Help Information on this (link here). Like I said, I have done this with the JS API to make D & E sized maps.
This is AGOL EB... I will try out your solution