Using the offline version, I leave the box that resizes the gui to the window size (checked by default in the magnifying glass tab) and resize the window to fit the character. I use export size 5.
By zooming and adjusting the position, I can get very tall or very wide images
(space+q or w really helps out here)
This works because offline Kisekae will attempt to grab everything in the window, including stuff outside the usual dimensions.
Attached is an example of me using this technique