Headlines with fixed size and positions are the ones which use range of allowed/required number lines in a paragraph style, and vertical anchor.

The main property of such headlines is that they feature paragraph grid (so you can see their structure clearly even if there are no placeholders), and that the lower headline elements (e.g. sub-headline) keep their position also in the absence of the higher elements (e.g. headline or kicker).

Edit such headlines as any other headline -click where you want and type the content. While writing, though, look at the grid and at the position other elements.