The 2D image export in Skup (Win) is always related to your used width-height ratio of your SkUp viewport - yes, this is very annoying. I also have this problem very often when colleagues work with my files.
If you render with any external renderer or any render plugin that all doesn´t matter since you're free to set your desired certain aspect ratio.
For exporting single images within SketchUp Pro (Win) you can use the Animation Export. There you are free to set your aspect ratio, regardless of your current Skup viewport sice.
Within the animation export you choose export "image series" (I don´t know the English naming here) instead of choosing export "animation"... (Unfortunately, this export function does not have the possibility to control antialiasing)
I hope this evasion helps...
PS: I do not know how SkUp(Mac) behaves in this matter
Animationsexportoptionen.jpg