Hi and thanks for your prompt reply.
Calibre is not showing any problems and, as you say, the covers display fine as they are automatically adjusted for display.
I have started using Calibre Companion on my Samsung Phone and Tablet (which is excellent by the way!) but I don't think the program as as sophisticated as Calibre. It displays differing sizes of book covers which made me look at the cover sizes in Calibre itself. Some were actually very large (3000x4000pixels) whilst others tiny which was obviously causing the difference in Calibre Companion.
That's when I decided to reformat the covers. Firstly to reduce the huge ones, saving space and secondly, to make Calibre Companion look better.
How I reformat them is by right clicking the cover picture in the Calibre Metadata Screen - Copy.
Open Paint.Net - Edit - Paste as New Image. (cover is displayed)
Click - Image - Resize - (enter pixel size (327x500) then close Paint.Net with Red X in right top corner. (I do this as I don't know which covers are going to need changing).
The Unsaved Changes Screen appears - click Save. The SAVE AS screen appears with File Name box and Save as Type box (with PNG selected). This is were I have to change to jpg.
The quickest way I have found of filtering through the covers is using the next/previous buttons on the EDIT METADATA screen and editing them through Paint.Net. I have to save the changed cover to a designated folder to then browse in the Change Cover area of Calibre to change to the resized cover.
As I stated in my post, I have no knowledge of programming and don't have any idea if what I suggested would even be possible. What I'm doing in Calibre might be a symptom of undiagnosed OCD but it keeps me happy! The original jpg's (as far as I'm aware) are stored in each individual book folder and would take forever to go through and even I'm not that compulsive!
If, with your expert knowledge of Paint.Net, there is another way of speeding up the process (which is of my own making anyway!) I would be exceptionally grateful!
Yours,
Woodgnome