This is more a question for the Technology/Computer section, but hey-ho!
To ensure that your picture file does not get reduced when sending on, it helps to check its format/size from scanning to sending. When scanning, make sure you save the picture as a TIFF or BMP file. (i.e. uncomressed.)
Rather than right clicking the file and clicking the 'Send to/email recipient', create a new email and then attach the file. You should then see the file size in the attacment box on the email, which should match the original TIFF or BMP size.
Add the addressee, subject and type any message required and send. Snags was quite right to ask because when you right click the file to 'send to...' very often a small window comes up to ask if you want the picture reduced before sending, in which case you would click 'No'.
Others will find this method a bit long winded and advise even more ways of doing it, but when you try to send a jpeg file it often gets reduced even further during the 'passing on' stages, without the sender being aware.
If, after doing what I suggest and your recipient still gets his/her picture arriving reduced, then it's the recipient's computer that's doing it. To see the result before sending it on, you could send the picture to yourself first or just look at your 'Sent file' list. Hope this helps.