Screenshots cannot have transparency and I'm not even sure why anyone would even bother. Simply render out a still frame from AE using Composition --> Save Frame as with teh background elements disabled and choosing a format that supports transparency like the default PSD.
...and unless your print standards are on the low side, it could be iffy you'd get the resolution necessary for print. Not even at 4K.
Here's what I mean: let's say you're working in 4K, which I'm pretty sure is 4096 horizontal pixels. You need to create an 11x17-inch poster at 600 DPI.... the "D" in "DPI" meaning "pixels". 11x600=6600. Your max is 4096. You can see that you may have to fudge a bit. The fudging gets worse if you're working in HD in AE; it maxes out at 1920 pixels across.
I'm not even sure why anyone would even bother
I am sure that I just tell why WHY in 1st post.
Thanks for a "save frame as" tip!
Dave. Thanks for tips & advices. Personally I never work on project bigger than 300 DPI.