convert photoshop file that breaks Mum's computer

We got a nice package in the post from Mum.
A .psp file.
It's is too big for Mum's computer, it just hangs forever trying to open it.

I double clicked on it (ubuntu intrepid + gimp) and it opened grand.
Open source and the gimp (free alternative program to photoshop) wins! :)
I plug gimp to Mum :) http://www.gimp.org/windows/
The .psp file is 340M.
Not absolutely ridiculously hugemongous but not neat.

saved as .jpg 5.6M 200mmx200mm (A4 width) 1600x1600 pixels http://www.dspsrv.com/~jamesc/photos/Visit%20to%20Mayo.jpg
sized down 0.5M 50x50mm http://www.dspsrv.com/~jamesc/photos/Visit%20to%20Mayo_Thumb50x50.jpg

me:
> Your file is quite big alright - your photoshop file is 340M.
> Probably it is so big as you have enough resolution to print it bigger.
> If you save it as jpeg at a fixed printing size you get rid of the
> extra resolution and you get a smaller file.
> I have saved it as a 5.6M jpeg which should print out on A4 okay and
> a Thumb for viewing. I have put them on the dspsrv.
> You can look at them there and right-click and select Save As ...
>
> The .jpg there is original resolution you sent 1600x1600 pixels = 200x200mm
> It is 5.6M in file size.
> The Thumb is much smaller 50x50mm = 544k (= 0.5M)
> If you printed the Thumb full size (A4) it would probably look blurry.
>
> A normal sized photo could be from .5M to 4M in size (depending on
> resolution).
> The higher the resolution the bigger the size and the bigger you can
> print it.
>
> Too much info probably.
> We need a diagram.

We need a simple web page on file sizes, photograph file sizes (compression, resolution and physical size) and on download/upload speeds. I can't find a nice one with a quick web grep.