You can reduce the display size of images with a simple mod to the first part of the image tag, just add a width or height number
(I used 800 as an example):
Base tag:[img]
Modified tag width:[img width=800]
Modified tag height:[img height=800]
The space between img and width or height is important.
Clicking on the image opens it up full size.
I modified your big pic to [img width=1000]