Import cover from file

First excuse my bad English.
I could not find any topic to help me.
I have thousands of folders with MP3 files. In each folder there are several covers of various sizes (in KB and in pixels), but what interests me are the layers that are below 50 KB or have 300 x 300 pixels.
What formula should I use the script "Import cover from file".
I thank you a lot for your help.

To add a specific file like folder.jpg, simply enter "folder.jpg" in the input box.
If you want to take the first picture file found in any folder, use *.jpg
If you have a a certain string (e.g. "front") in the filename, use e.g. "front.jpg"
MP3tag does not look into the file properties of a picture file.
That means: you cannot select by filesize or dimensions.