Quicktime 10 - export in original size, not 4k, 1080p, 720p etc?
There is no way to export in any other sizes other than 4K, 1080P, 720P or 480P, according to this post here (it's in Mandarin, but I translated it) You could, however, export it in a higher resolution (such as 4K or 1080P) and then use another program to save it in your preferred recording.

Why do you need these strangely sized videos, by the way?