How to modify margin spaces for Visio drawing when using ‘Auto fit to Drawing’

Found it just to un-linking any reference to the data source, right-clicking the data source pane and choosing Remove Data Source Link.

The recommended way of doing this to create a new Template that has the settings you want in the document's "styles".

First, create a new blank drawing.

Then, open the Drawing Explorer (This is on the Developer tab on the ribbon. If don't see the Developer tab on the ribbon, you need to enable it in Options > Customize Ribbon.)

In the Drawing Explorer, expand the Styles node.

Right-click on Normal and select Define Styles

The Define Styles dialog will appear.

Edit the Text, Fill, or Line properties as needed. In this case I illustrate making the default line color red.

Save the diagram as a Template (VST nor VSD) into a folder. Typically I make my folder under c:\users\myusername\Documents\My Visio Templates. Give it a nice name like "My Default Template.VST"

Close the document.

Go to File > Options > Advanced. At the very bottom click File Locations.

Set the Stencils value to C:\Users\myusername\Documents\My Visio Templates

Now from File > New you will see "My Default Template" in the The Template Categories list.

Just click on that template, and now any new documents will have the style

Now when you try to draw a shape, the default line color will be red.