Form size to fit most monitors

What would be the best form size (height, width) for a form to fit most monitors and tablets while the text is still readable? A4 landscape or something 16:9?

What I do here:
Set Form to maximized (Macro).
Make a screenshot of the form, which is filled with most content.
Open this with GIMP and see width and height in pixel.
Zoom all forms so the form will fill the whole screen (Macro).

Thanks RobertG. I tried your method but the text looks not clear in GIMP full-screen mode. Resolution must play a roll here too. My two monitors are 1920x1080 and 1440:900.

Not “GIMP full-screen”. Full screen will be set later in document. Screenshot of the form will be made with zoom 100%, saved (or copied to) GIMP as png. Then I will shrink this image to the “used area” and blow up the zoom by macro to the used screen.

Macro will work like this:

SUB ToolbarsHide(oEvent AS OBJECT)
	DIM oFrame AS OBJECT
	DIM oWin AS OBJECT
	DIM oLayoutMng AS OBJECT
	DIM aElemente()
	DIM inx AS INTEGER
	DIM iny AS INTEGER
	DIM inDpiX AS INTEGER
	DIM inDpiY AS INTEGER
	DIM inWidth AS INTEGER
	DIM inHeight AS INTEGER
	DIM inZoom AS INTEGER
	oFrame = oEvent.Source.CurrentController.Frame
	oFrame.setTitle(oEvent.Source.Title)
	oWin = oFrame.getContainerWindow()
	oWin.IsMaximized = true
	oLayoutMng = oFrame.LayoutManager
	aElemente = oLayoutMng.getElements()
	FOR i = LBound(aElemente) TO UBound(aElemente)
		IF aElemente(i).ResourceURL = "private:resource/toolbar/formsnavigationbar" THEN
		ELSE
			oLayoutMng.hideElement(aElemente(i).ResourceURL)
		END IF
	NEXT	
	inWidth = 1360
	inHeight = 800
	inDpiX = Int(1440 \ TwipsPerPixelX())
	inDpiY = Int(1440 \ TwipsPerPixelY())
	inx = Int(oWin.Size.Width * 100 * 96 / (inWidth * inDpiX))
	iny = Int(oWin.Size.Height * 100 * 96 / (inHeight * inDpiY))
	IF inx < iny THEN
		inZoom = inx
	ELSE
		inZoom = iny
	END IF
	ThisComponent.CurrentController.Sidebar.Visible = False
	ThisComponent.CurrentController.ViewSettings.ZoomValue = inZoom
	ThisComponent.CurrentController.ViewSettings.ShowRulers = False
	ThisComponent.CurrentController.ViewSettings.ShowParaBreaks = False
END SUB

inWidth and inHeight are the values I will get from GIMP
Macro will be bound to Formdocument → Document open (another thread these days for Base here).

Am not concerned about functionality and try to stay away from macros for now. Just designing Base-forms to be logic and attractive for the user and to contain all fields and buttons.
Have now set my page size to 16 x 9 cm landscape. Then I save my form as PDF. Open the pdf and view - fullscreen. With this setting I can read text in 6pt on the smaller screen (tablet) well enough.