So currently there is a bug with rendering at sizes over 750-ish in the height. https://github.com/synfig/synfig/issues/755
My current work-around is to keep my canvas at 1920x1080, but render out to 1280x720 for the moment until the bug is fixed. However, when I specify 1280x720 for the canvas Width and Height in the Render dialogue, it just straight crops the output to that size, rather than scale it to that size like I hoped.
Is there any way to do this? (Other than re-work the entire animation to downscale it, then re-work it again to upscale it when the bug is fixed!)