Sizing of photos

Hi,
I desperately need help with this problem as it has bugged me for ages.
Basically, whenever I send a photo either via email or across instant messenger it turns out really tiny and pixels out if the person tries to make it the original size.
Also, I want to order prints from truprint but when I upload the photos from iphotos they upload and truprint has an error mark saying they are too small to print in good quality. My photos are taken on 5-7 megapixel cameras so I know they are good size and in iphoto they are fine.
Is there a way to change settings on how they send or am I doing something wrong?

Please help!

When you export or email (you ARE going through iPhoto, the app, aren't you?-- you should be), you should be prompted to indicate what size you want them to be. What are you choosing there?