MyBB Community Forums

Is there any possible way to resize forum icons through the use of a plugin such as MyForumIcons?

I doubt that. If you want to resize any forum icon you can download the icon into your device, resize it with any image editor then upload it one again with the same name and extention it had when you downloaded it.

Or

Go to the template set where the icon is used if it contains an img code the resize it using the width and height line of code.