Loz
Mongoose
That's fine for offensive purposes, you need to know how big the attacking weapon is in order to determine if you can parry it effectively. But for effective size when parrying, it needs to have some kind of special case like the Military Flail.
Why? A weapon is an extension of the limb holding it. A gorilla lashing out to strike with its fists has size M for its mass, but also size M for parrying purposes this doesn't change. Why would you introduce a special rule?
A military flail is clearly an exception due to the way the weapon is constructed: a highly flexible damage delivery end attached to a rigid haft. When attempting to parry with the haft of a flail you treat it as size M rather than size L - but this is a function of its construction.
If you throw-up an arm to ward off a blow you'll instinctively throw the entire arm in the way of the weapon because that gives you maximum potential protection. An arm is probably of the same length as a broadsword's blade (which is the bit of the sword doing the damaging) and certainly has a thicker profile (although not nearly as tough, unless encased in plate armour). And, even though an arm is jointed at the elbow you actually have two rigid portions available - not a rigid portion attached to a highly flexible, dangly one.