Lower legs, from the knees down, are where most severe bleeding happens because their major blood vessels are close to the surface. Wounds over a tendon or a joint can be serious because of the structures they may involve. Arteries higher in the body are buried deeper and are more protected. Wound size doesn't always equal severity. Small puncture wounds can be much more dangerous than big ones because they're hard to clean completely, so bacteria can get trapped inside.
Time is critical.
A wound treated and sutured during the first six hours after it happens has the best chance of being cleaned out completely and not becoming infected. That's why I advise calling your veterinarian before you do anything else. You can help the process along by applying pressure over the wound, giving the clot something to form against. Ideally, you do that with a pressure bandage. If the wound is where you can't wrap it, such as high on the body, steady manual pressure will do the job until clotting shuts off the flow or help arrives.