As I was posting the idea, I came across another idea that had already been posted. Not sure why this didn't pop up the first time when I searched for the topic... I love Marketo, but sometimes it doesn't keep things as simple and intuitive as it could!
You can do it just by adding a line of code:
<div class='cf_widgetLoader cf_w_mywidget' options='{"outerHeight":400, "outerWidth":600}'></div>Here's the link:
https://community.marketo.com/MarketoIdeaDetail?id=08750000000IRnLAAW You can apparently also do it by dragging the corner in the editor, but again... not very intuitive!