Nah, nothing - well, almost nothing - is too hard to handle with forum BBCode! Just add the desired dimension(s) as parameters in the opening image tag and it'll scale for you:
To get this size image, I used the following BBCode:
[img width=1100]http://pinkhandguns.com/wp-content/uploads/2011/06/bulletcalibercomparison.jpg[/img]
The dimension(s) you want go in the opening tag, in the form {dimension}={size} with the value of the size being in pixels; e.g., in the code I used, the width of the image is set at 1,100 pixels. If only one dimension is specified, the software will automatically scale the other dimension.