RectF.Stretch (gb.geom)

Static Function Stretch ( Width As Float, Height As Float, Frame As RectF [ , Alignment As Integer ] ) As RectF

Stretch a rectangle so that it fits inside a target rectangle frame while keeping its proportions, and return the geometry of the final rectangle.

Width and Height are the size of the initial rectangle.
Frame is the target rectangle.
Alignment is one of the align constants that defines the relative position of the initial rectangle inside the target rectangle. By default it is centered.