First of all, terminology is hard. Which is to say, it can be difficult to match a name that is in an inventory or chronicle with a weapon that is depicted in an illumination or an artifact that survives in a museum. This is because when we are studying weapons that have fallen out of active use, they have similarly fallen out of our vocabularly. Conversely, people mentioning them in things like chronicles an inventories know what they meant and expected their contemporaries to know as well. A further complication is that when we talk about European polearms from say, 1000 to 1600 we are talking about a span of 600 years across an entire continent. Terms came to mean different things over time, even in the same language, and different nations had different words for the same weapons (the Italian Roncone is essentially an English Bill, though their designs differ in the particulars). You have seen the results of this confusion yourself in the varied descriptions of what a Guisarme is.

I have not myself seen definitive evidence for what sort of weapon is accurate described as a 'Guisarme'. In the Musee de le'Armee in Paris, they have a series of Italian bills from the late 15th and early 16th century (Ronca/Roncone, to use the Italian term) labelled as 'Guisarmes'. You can see the same in the Metropolitan Museum of Art. This is also the 'Guisarme' illustrated in many roleplaying books, like the (in)famous Polearm spread in the DnD rulebook. Does this reflect a genuine medieval French usage of 'Guidarme' for Bill? Or did 19th century collectors decide that 'this is a Guisarme'? The exhibit does not provide the evidence to judge. In any case these bills would be mounted on hafts around 6 feet long. Their heads would be a bit over 2 feet long and weight over 2 pounds, according to Waldman.

The other possibility is that a Guisarme is a kind of long-hafted cresent-bladed axe, where the bottom part of the blade is fastened to the haft. On the whole this weapon is rather like the later Eastern European Bardiche. This is the definition used by John Waldman in "Hafted Weapons of Medieval and Renaissance Europe." Waldman offers little direct evidence and instead refers to an article in a journal from 1974 by A Borg. Waldman does note that in English 'Guisarme' is translated as a 'Scottish Hand Axe' in the 14th century, which does suggest that unlike the bills noted above it is an axe-like weapon. He also notes that in 'Sir Gawain and the Green Knight', both Gawain and the Green Knight are noted as using 'Gisernes.'

And now for a digression into weird Middle English Poetry. The Epononymous Green Knight in the 14th century poem is armed with a 'Gisarme' and some illustrations show wielding wielding a crescent-bladed weapon, like this 14th century illuminated manuscript from the British Library. The 'Gisarme' of the Green Knight is also called an 'axe' in the text - the terms are used synomously.

The original description is of an axe with a very large blade (as per A.S. Kline's online translation, found here):

and an axe in his other[hand], one huge, monstrous,

a perilous spar to expound in speech, who might.

The head of an ell-rod its large length had,

the spike all of green steel and of gold hewn,

the blade bright burnished with a broad edge

as well shaped to sheer as are sharp razors.

The shaft of a strong staff the stern man gripped,

that was wound with iron to the wand’s end,

and all engraved with green in gracious workings*

Now keep in mind that the Green Knight is a supernatural and allegorical figure. The 'Ell'-long blade is either around 24 inches long or over 40 inches long, depending on the Ell that the poet is referring to here. So this may be a hyperbolic description of a supernatural figure's weapon, or a fairly believable blade length. We can be sure that it is a) an axe-like weapon, and b) has a long blade, not a short one like some other types of war axes and polearms (IE it makes sense to think of this blade in terms of extreme length when you are telling a story like this - you wouldn't describe a halberd this way).

Later, when the Green Knight offers his axe to whoever shall behead him (and be beheaded by him in turn...like I said it is a weird poem), the axe is explicitly referred to as a 'Giserne'. Translations generally do not include the term, so I am using the original text. The funny looking letters are thorns - they make a 'th' sound:

"I schal gif hym of my gyft þys giserne ryche, Þis ax, þat is heué in-nogh, to hondele as hym lykes,"

So the Green Knight's Ax is a 'Giserne.' Combined with the illustration above, this is a decent argument for Waldman's interpretation of a guisarme as a cresent-bladed axe like weapon. So what where they like?

Again as per Waldman, early Guisarmes have blades of between 45 and 60 cm (so around 18 inches to 2 feet). Later examples from the 16th century have blades a meter long. These also can have reinforcing straps (langets) around the shaft (like the Green Knight's, though not 'to the Wand's End'). The shafts are around 4-5+ feet in length. Unlike pollaxes, these weapons do not have back spikes, and they are not always suitable for thrusting (depending upon the shape of their blade).

Now this is good evidence but it doesn't demonstrate that other people at other times didn't call something else a Guisarme. Perhaps this usage is particular to 14th century England, and the term means something else elsewhere. After all, Eastern Europeans of a slightly later period called very similar weapons 'Bardiches'. Like I said. Terminology is hard.

Sources: As cited in-line. Much of this is indebted to Waldman - his notes about the term allowed me to find the sources above and reconstruct a fuller argument for its use.

*The original reads:

& an ax in his oþer, a hoge & vn-mete,

A spetos sparþe to expoun in spelle quo-so my3t;

Þe hede of an eln3erde þe large lenkþe hade,

Þe grayn al of grene stele & of golde hewen,

Þe bit burnyst bry3t, with a brod egge,

As wel schapen to schere as scharp rasores;

Þe stele of a stif staf þe sturne hit bi-grypte,

Þat wat3 wounden wyth yrn to þe wande3 ende,

& al bigrauen with grene, in gracios1 werkes;