Paul, if you want to change embedded video size, first of all make sure that Disable Custom HTLM is unchecked. Go to Admin Area > Settings > Content settings page.
Then post your embed video code in Forum using HTML button, and change video size directly within code.
I've checked our Demo and everything is working properly.
So, to change the embed video size you don't need to change the Video plugin settings. These settings will not effect the embed video size posted in the Forum. They will change the size of the player displayed on the video page.