I'm a long time Geocortex Essentials admin who is beginning to transition to ESRI web maps.
In the Print widget, it offers the default layout sizes: A3 Landscape, A3 Portrait, A4 Landscape, A4 Portrait, Letter ANSI A Landscape ,Letter ANSI A Portrait, Tabloid ANSI B Landscape, Tabloid ANSI B Portrait, and MAP_ONLY.
In Geocortex, I am able to make my own templates, put the title, legend, infill map, scale bar, etc. wherever I want...all built Reports/Designer page and easily offer them up as choices for clients to pick. I was also able to make them based on true printer sizes such as 11x17 Tabloid Landscape, 8.5x11 Letter Portrait, etc. which is what our clients are used to and want.
So far in WAB, I keep asking myself...where is this ability? I have scoured GeoNet and other internet pages and I see so very little about adding custom sizes or altering existing ones. In WAB, in the Print widget, it offers no ability to make any changes. I'm sure it's out there but I've yet to come across it. :)
Am I stuck with the defaults?
here is the doc we have about authoring a print service with your own custom templates.
also, pls see this tech sessions videos:
hope you will find this helpful