My most common use case for using Marked 2 was exporting Markdown as a statement (half US Letter) sized piece of paper that I would then pass to create booklet to print a little book. I am a pastor who writes 3 manuscripted sermons every week, and this is my prefered way of having my notes.
It seems in Marked 3 there is no way to get a statement sized paper. I tried setting it up in Page Setup, but the custom page size I created doesn't show up in the new print dialog.