Yes there is. Go into the page layout where the embedded dashboard page resides. Click on the wrench icon(Properties) and update the height accordingly. I have a task to making that piece a bit more dynamic. Regarding width, note that Salesforce places a max width of 1000px so I would suggest setting a max width of 955px or so to account for scrollbars if they're needed. Hopefully that does the trick.
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Just as an additional note. I had to set both width and Maxwidth to get mine to size the way I wanted it to. Even if you set Maxwidth to "infinite" but do not define Width it will default to the default width of 800px
*edited to reflect the correct default Width
I originally misread your post. That's good info to know. Thanks for sharing.