From Xojo Documentation

Property (As Integer )

aWebPage.MinimumWidth = newIntegerValue
IntegerValue = aWebPage.MinimumWidth

New in 2010r4

Supported for all project types and targets.

The minimum width of the WebPage. While we cannot prevent the user’s browser from resizing too small, we can prevent the content from doing so. If the browser window is smaller than the content, scroll bars will appear.