I have used one of my databases to test my code I use a text box called text20 that holds either yes or no and uses it to switch sizes.
My image is called photo1 and this code changes the size and position, note that the dimensions use twips in the vba code so you will ahve to play around a bit to get your image the sizes you want 1" =1440 twips.
You will probably want use the code in the forms On current event to set the default size.
If Me.Text20 = "yes" Then
Me.Text20 = "no"
Me.Photo1.Width = 2880
Me.Photo1.Height = 2880
Me.Photo1.Top = 1000
Me.Photo1.Left = 5000
Else:
Me.Text20 = "yes"
Me.Photo1.Width = 5000
Me.Photo1.Height = 5000
Me.Photo1.Top = 1000
Me.Photo1.Left = 4000
End If