Hi Simone,
Sorry that I've been away from my computer for a while.
The Windows explorer page usually opens the Windows Photo viewer, which is a very unhandy program for changing image size. If I recall correctly, you
can specify various pixel sizes to send in an email by right clicking on the image and that's about it.
Most folk use programs that are designed with image processing in mind, rather than just for viewing them . The popular ones are Lightroom, Photoshop (which I use myself) , DXO and Photo Ninja. Cameras generally come with a free program like Nikon's Capture NX-D or Canon's Photo Processing software. and these are free. There are other good free programs and I recommend that you download a free program like
Gimp , a program which photographers like Peter Gower swear by, and I understand isn't difficult to use.
Our main site has lots of info on how to resize images, both in terms of their pixel size and the quality settings. Our illustrious web master, Rob Parker, has a section entitled Resizing and Cropping your Image which explains the principles very well. I'm having trouble finding it in the library at the moment, but will keep looking. i seem to be having a bad hair day.
If we're lucky, maybe Rob will jump in and give us its web address. I'll post it myself when I find it.