Thumbnail size (for the actual Thumbnail mode*) isn't part of the folder format, although I think that's on our ideas list.
There's a global size setting, and it can also be changed in the current window on the fly via the toolbar or via commands.
But you can use a script to change it in particular folders:
(* You can also have a thumbnails column when in Details mode, which can be resized freely. Since that's just a column like any other, its size is saved as part of the folder format.)