The Stimulus Editor will allow you to work with any tablet size, logically.

When you have a stimulus open (new or existing), go to the Workspace section and select "System".

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In the "Properties" window, the display and tablet sizes will be displayed. Simply click on the value for the height/width of the tablet, enter a new value and hit ENTER.

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The grid in the editor window will be adjusted accordingly, and all elements will be drawn according to the new scale.