Awesome thanks! Thought I was misunderstanding it! Sorry I was away for a few days at a LAN and didn't check mail.
You still have a little bug there - if you go up to say 1000 then drop it back down to 500, you get a black box around the outside of the preview window until you save and re-load. It's a minor thing though. Screenshot below.
https://drive.google.com/open?id=1AyXT7YJuD02bxZYftyHhtqIzrrDT18lK
Also again a very minor issue but at the larger preview sizes, you get flickering. I think it's if the preview hits the edge of the screen perhaps so the fix for me is to just lower it a little and actually I wouldn't have it that large anyway so it isn't an issue for me but thought i'd report it.
edit just found the cause of this - it has nothing to do with the preview size but it's when your mouse rolls over the preview window. It causes it to flicker quite heavily. Presuming it's all zoom levels. I'm on 500 preview size and it does it. I'm on the latest version I think.
Cheers