If you create a panel on a form and set it to Dock=Top and drop another panel and set its Dock=Fill, it may fill the entire form, ignoring the first panel. Changing the tab
Another, potentially cleaner option is to use the TableLayout control. Set up one row of the desired height for your top dock, and another row to fill 100% for your bottom. Set both panels inside to Fill, and you're done.
(TableLayout does take some getting used to, though.)