F-clamps are available in many different sizes. Always use an F-clamp which is large enough to support the workpiece, otherwise there will be a risk of damaging the screw.
Similarly to the majority of clamps, an F-clamp’s size and clamping capacity is determined by its jaw opening and throat depth.
Jaw opening
The jaw opening of a clamp is how far the mouth of the jaws can open.
The jaw opening capacity of an F-clamp depends on the length of the bar.
The smallest jaw opening available is 80mm (3″ approx.).
The largest jaw opening available is 1500mm (59″ approx.).
Throat depth
The throat depth is how deep the jaws of a clamp are, and is measured from the top edge of the jaws to the bar.
The smallest throat depth available is 50mm (2″ approx.).
The largest throat depth available is 175mm (7″ approx.).