The right head/line size for your rod may depend on your casting style, personal preference, or other unique situations. The best place to get that dialed-in is with your local fly shop.
Fast action rods feature softer tips than medium action rods and provide a ‘stiffer’ feel. Fast action rods excel in the hands of anglers with more aggressive casting strokes and greater stroke control. Medium action rods are ideal for anglers with more fluid casting strokes and have a larger ‘sweet spot’ for a wider range of casting strokes. Consistently throwing a tailing loop? Your rod’s action may not be a good fit to you and your casting stroke.
A rod’s power is its resistance to flexing under load. Powerful rods are ‘beefier’ through the bottom half, generate higher line speeds, and are more adept at keeping big fish pinned. Delicate rods are more presentation-oriented, protect light tippet, and are a better choice for picky fish.