Here is my original image, width 1000, height 667:
Here is the same image with size modified by adding "width=500 height=333" after "[img" at the start of the code
Like this...
img width=500 height=333]http://i1287.photobucket.com/albums/a621/GrahamB2013/IMG_5000a_zpsofuagpnp.jpg[/img]
(I omitted [ at the start so the software treats this line as text)
Note- the width and height figures must stay in the same proportion as for the original picture, or it will come out distorted, like this (width 500, height 600)