I've thought about the issue of customisable dimensions today. I don't know how good configuring each media site individually is, considering the add-on supports nearly a hundred of them. Right now what I'd like to do is have a customisable min/max width and automatically adjust the size of each embed with the correct aspect ratio. That comes with its own set of challenges (some embeds such as Twitter's tweets don't have a fixed ratio, their height adapts to its content) and I don't know how much work it will require yet. I'll update when I know more.