I suggest that, when editing a master item visualization, the size of the edited object can be set to whatever size is desired instead of automatically taking up the entire canvas.
When editing a master item visualization, it is automatically expanded to use up the entire canvas on the screen. This makes it impossible to view the master item within the sizing context of where it is expected to be applied, and as a result, means it's difficult to get it looking as expected, particularly for smaller visualizations.
The only existing options for this now are:
* Edit the master item, then click "Done", then look at a sheet where the master item has been added and sized correctly, and then repeat the process until you get it looking as expected.
* Resize the browser window until the canvas is the same size as the object is eventually expected to be. This is tricky to do and requires constantly changing the browser window's size as you switch between editing different objects or viewing the sheets.