I doubt you can expect a browser to be a certain size, and I agree that making the tablet design look good on a desktop is difficult.
Have you taken a look at "layout renditions" in Layout Templates?
I typically start by designing for tablet, then you can add layout renditions for phone and web viewer. This might help.
My process (developed with Bob Bringhurst) is to create a framework of controls (add the cards I expect to use but don't design them, then add the mapping rules that will control card use) and eventually "design" things by adjusting the properties for the layout and cards. The advantage of this approach is that I get to see a variety of cards in the layout view and adjust as I design. It's a little blind initially, but we like the approach.
Once the tablet is working the way we like, we add a layout rendition (for phone or web view.) And then we adjust the layout and/or cards to work on the different device/desktop class. All setting from the tablet layout rendition are copied into the new rendition, and then changes are made.
This should help with controlling the web viewer experience.
HTH