The forum width message window will allow for images up to 490px. Any image that exceeds this max width will create a horizontal scroll bar allowing the image to be viewed in full but maintaining the message view width. Here’s an example:

Additionally by default, if you click on the:

you get:

Which by default limits the size to 470px. well within the forum width requirements.

If you image is smaller then 470 it will still make it 470px unless you manually change it. you can always remove the size=470px portion of the tag. just be share to leave no space then. so you’d have [img]

The smallest you can resize the [img size=(number)] tag to is 100. anything smaller just remove the size=(number) all together.