It's a bug with the new settings menu that you can't edit the avatar size unless an AV chat tech is enabled. Once AV chat tech is enabled, you can find the option under the video display option of audio/video: EDIT: You don't have to leave the AV chat tech enabled for your selected option to be used, only to change which size you are using. Also, enabling the chat tech can only be done by the GM (and maybe only by the game's creator; not sure on that part).
