The clipdist is only used for sprites that are not flat. If it is flat, the game would use the sprite's dimensions as its clipping boundaries.
If the sprite is not coded with a clipdist, it will use the clipdist as shown in the map editor (default is 32). Keep this in mind, as a sprite's clipdist is independent of its size. The player and projectiles can easily pass through a large sprite if its clipdist is too low.
The clipdist values for certain hard-coded actors are shown below:
All bosses (palette 0): 164
All enemies: 80
Most props: 32
BLIMP: 128 (The large blimp in E3L9 is difficult to hit because of this low value.)