Each form has their own instance of the class, so they wont get each others values. I don't know where the subscript error is coming from (but then I haven't looked at the class yet).

If you resize the MDI Parent, the child forms will only resize if they are maximised. If their WindowState is Normal, they will stay the same size (and the parent will get scrollbars if apt); if you want to change that, you will need to add code to the resize event of the parent (check if Me.ActiveForm.WindowState is Normal, if it is then resize the child forms as you want).