Sprint, middle-distance and distance spikes are three different tools. Here's how to pick the right one for the events you actually run.
Spikes are event-specific. A sprint spike has an aggressive, rigid plate and almost no heel; a distance spike is softer, more cushioned and far more forgiving over 25 laps.
If you race across a range — say 800 m up to 3000 m — a middle-distance spike is the sensible single purchase. It handles the speed of a fast 800 and stays tolerable through a 3000.
Fit should be snug with no heel movement and no toe pressure at the front. Spikes stretch slightly with use, but they do not change size.
Pin length is dictated by the track, not by preference. Most synthetic tracks allow 6 mm; many facilities cap it, so check before you race.
Finally, introduce them gradually. A few strides and one short rep set in your first sessions is enough — calves and Achilles need time to adapt to the drop.