I'm going to reference "that setting" a few times in this post. It refers to the setting: File Display Modes > Thumbnails > Appearance > Size.
Infotip thumbnails (set through Settings > Filetypes) for text files will not display larger than that setting.
Images file infotips (I tried JPG and PNG) will display larger that setting.
Also, if a text file's infotip thumbnail is set to smaller than that setting, the text gets scaled down (making it impossible to read, depending on the ratio of the infotip to that setting).
If anyone has time -- this is still an issue for me.
I have to move the slider down every time I need thumbnails view, because I keep it set to 512, so all my text files will show infotips that size. If I forget to move the slider back up to 512 before I leave thumbnails view, then the next time I need an infotip, I have to switch to thumbnails to run the command
Show THUMBNAILSIZE=Reset
because the command won't run if you aren't in thumbnails view.
I guess not too many people care about this, so I'll understand if it never gets changed.
You can configure the infotip to show thumbnails at any size you like; e.g. if you change the code to {thumbnail:1:512} it'll show 512 pixel thumbnails no matter what the actual thumbnail size is set to.
It's not working here.
Open a dual lister, change one side to thumbnails view, and in the thumbnails toolbar click a size smaller than than 512. Then in the other side of the lister hover over a text file type that has {thumbnail:1:512} in it's infotip -- the infotip thumb is constrained to whatever you changed the thumb size to in the other side of the lister.