Use the Shape.ScaleHeight, Shape.ScaleWidth Methods
Sub ChangeShapeSize()
Dim Ht As Single, Wd As Single
Ht = 1.75
Wd = 2.75
With Worksheets(1).Shapes("CheckBox1")
.ScaleHeight Ht, msoFalse, msoScaleFromTopLeft
.ScaleWidth Wd, msoFalse, msoScaleFromTopLeft
End With
End Sub