To create a splitter (dragable border between controls) that automatically resizes controls on the form, you can use a label control. Access does not provide a splitter control and if you don't want reliance on external dependencies you can use this technique to create an all VBA splitter control.

The label can be set in colors that hide it on its background, and then you can use its events to create the actual dragging and resizing behaviour.

An example using this technique can be found here. It also works for Excel and other office apps.