How to make rendered image look like the preview?

Well, it depends on the source image you are using

I may suggest two ways to do it

1 - The easiest one, is to make an capture area screenshot of the preview when is set to reduced, because it is really a 600x600 pixels image, so the screenshot would have the same quality and look

2 - Reduce the source image you are using to 600x600 pixels (or 600x any ratio pixels size if it is not square) and try the filter and see if you could get the same result rendered, maybe it will not give the same result as the one you have when using the real size image, but you can try and see what you get

if you need to have it at more than 600x600 pixels you may be able to make a zoom of the screen and then use a software that is able to capture a screenshot of a zoomed screen so it would have higher resolution at least in theory.