I've tested this on two seperate computers and it would appear that the actual size limit to a Bitmap in Firemonkey is 8000x8000 px meaning both Width and Height respectively cap at 8000 px size.
This is what I observed, if anyone gets a different result please let me know.
How you plan to get around that is up to you, I would suggest dissecting the source image into multiple parts so that neither part excedes the limit and then assign each part to a different Bitmap component(such as TImageViewer) and then make it all come together as a whole.