I have a large activity diagram, which grew more than 4096 pixels tall. I noticed first that the bottom of the diagram was missing. This can be fixed by using the scale command, which allows the whole diagram to fit nicely to the size given. Unfortunately the fonts and diagram elements start to get unclear (pixelated) when the diagram gets very large.
I tried to use a number which is bigger than 4096 in the scale command limits, but the output was clipped to 4096 picels high.
My version is: PlantUML version 8034 (Sat Jan 09 12:07:59 EET 2016)