Filters Larger than 600p

Every filter can be used with any size.
The 600x600 pixel is just the default when you start FF.
To make a filter work on a larger size, simply go to:
File > New Image

In the dialog you can set the size you want. The filter will automaticaly start rendering at the new size.

And as the message says, you can go there to CHANGE the preview size to see it bigger than 600x600, BUT be aware that then FF will probably be much slower (depending of course the kind of filter you have choosen) as it will need to make a bigger result, and sometimes on slow filters it will keep all blank for a few minutes until you see something appear.

WHAT YOU SEE IS NOT WHAT YOU WILL GET

ALSO although you see in the preview the reduced 600x600 preview this IS NOT really what is important, the real result you will get depends totally on the resolution that is configured.

So you can keep previewing in reduced size at 600x600 a photo that is really for example 3000x2000, and DO NOT need to view it at actual size to be able to render the result at the real resolution that is configured. I mean that if you are in the reduced view, you can go to "SAVE AS" and the result will be done at the 3000x2000 resolution and NOT at the 600x600 you are viewing.

This is a very in-depth answer and also very helpfull:) Thank you guys!