You're right that the size of the map doesn't matter, although the aspect ratio does if you want the entire map. If you are happy with the wallpaper being a cutout of the complete map, then it is less important.

What I would have done is making a map with the appropriate aspect ration (For example for a small dungeon map, 192 by 108 may work). Then, I would have used the jpeg (or png) export option. Set the width/height to 1920/1080 (You don't want the final image to be larger than what your screen needs, that eats up extra computer memory for no good reason), then make sure "crop image to aspect ratio" is OFF and "restrict to map border" is ON. For a better quality image, you can turn on a bit of antialiasing, 67% equals the quadrupling of image size you had manually specified in your screenshot. Now, just save the image and it should be in the appropriate format.

That's intentional for the traveller symbols. The gas giant symbol isn't a primary planet in it's own regard, it is an indicator that a gas giant is present in the system, just as with the bases, and other information, basically just a modifer for the main planet symbol.

Just a general comment about live streams, YouTube always needs to process them after they are done, so it is out of our hands when the video will appear, but usually it will be a couple of hours after the stream ends.

You could always write such a command. An excellent candidate for a small XP project...

This is a side effect of the Delayed Drawn Symbols system. Symbols are redrawn at the end of the pass. However, the countertops, being polygons are not.

Two ways to fix this is to either set DELAYDRAWSYM to 0 to stop this behavior, or do as Jim suggest and use a dedicated sheet for the counters.