If you set AutoSize = true for a form, the default setting for AutoSizeMode is GrowOnly.  This means an AutoSized dialog in this mode cannot snap back down to a smaller size.  Typically this is not what you want.  If you want a resizable dialog that can stop at the PreferredSize, use the MinimumSize property.

this.MinimumSize = this.PreferredSize

Alternately, if you do not want the dialog to be resizable, switch AutoSizeMode to GrowAndShrink.