Replace "[" and "]" with "<" and ">", respectively. Also, replace 640 and 480 with the desired values. It's a HTML code.
[img src="image_link" width="640px" height="480px" *alt="image_description"*]
*You may omit that part of the code.
The best option would be for you to resize the image locally, through a software like GIMP or Photoshop, and later host the resized version. Think of this: you have an image like - 1400x2000, 891 KB. You may shrink it through a code, but both you and other users would still load the huge 891 KB file.