I agree the problem is related with cache.
I started Calibre, and don't doing anything more, run the plugin.
The sizes were fixed in eight books.
I ran it again... and detects only one book. It can't fix it... and it is the first book in my library... the one that is selected in the moment that Calibre opens.
Just to check... I change the "columns to sort the booklist by on startup" that way the first book in the library changes. Restart Calibre, and run de plungin: one book detected. Run again, and all sizes are correct.
I use Portable version, on XP SP3. I hope it can help...
PS) Theducks, thanks for the advice