On my main form I had set the AutoScroll property to TRUE and this seemed to work horizontally but not vertically.

I have a customer that has a laptop with a low resolution. Initially the vertical scroll bar would not appear and I set the AutoSizeMode to "GrowAndShrink" on my main form. That works perfectly.

I looked at the property on the main form AutoScaleMode which by default is Font. The other options are None, DPI and Inherit. When would you want to use these different options?

The AutoScaleMode tries to re-size the form according to the system DPI.  For laptops, Surfaces, etc, which have very small, high resolution screens, the DPI is often set to 125% or 150%.  The AutoScaleMode property then tries to increase the size of the form to match the increase in the size of the text.  I don't remember the intricacies of which option does what.  You'll have to try them out by setting the different options and changing the DPI on your computer to test it.