A pipe wrench grips a round section by biting into it, which is why it works on pipe where a spanner has nothing to hold. The jaws are hardened and toothed, and the grip tightens as the handle is pulled, so the harder the pipe resists the harder the wrench holds. Jaw capacity is stated per wrench, since a wrench that will not open round the pipe cannot grip it.
That self-tightening action only works in one direction. The wrench is pulled so the jaws close onto the pipe, and pushing it the other way simply slides the jaws round without gripping. Reversing the wrench rather than the pull is what changes direction. Some wrenches carry a reversible jaw so the direction is changed without turning the whole tool.
The teeth mark the pipe permanently, which is acceptable on a run that will be lagged or buried and not on anything finished. Where the surface has to survive, the pipe is protected or a different wrench is used entirely. Pipe that will be visible is usually gripped through a cloth or a soft jaw instead.
Length gives both leverage and jaw capacity, since a longer wrench opens further as well as pulling harder. The range runs 250mm on the Stillson pattern and 300mm, 350mm, 450mm and 600mm on the adjustable bodies. Send the pipe diameter to 07754288583 on WhatsApp. Capacity and length are stated together so the two can be read against the pipe.