In the early Thor comics, Mjölnir is portrayed as a sort of rough hunk of metal atop a three-or-four foot long wooden handle, with proportions similar to a sledgehammer, like this.
But in current Thor comics, the hammer's head is larger with a shorter handle, like so.
And here's the version from Ultimate Marvel. As you can see, it resembles a hybrid of an axe and a hammer.
So which of these most accurately resembles actual Viking weaponry? Were warhammers used at all in battle, or were they deemed too unwieldy?
I think I can answer this. Look at this drawing, and this picture. The first is a drawing of necklace found in Sweden, and the second is a picture of a runestone also located in Sweden. Both images shows the hammer Mjølner(Mjölnir in English). The hammer is rather small compared to that of the comic. Here's a picture of the most commonly found shapes of the hammer in archaeological findings of necklaces, jewelry and such. It does not resemble the hammer in the comics at all. In addition we have the account in Norse mythology. The Younger Edda by Snorre Sturlason (which is our chief account of Norse mythologi), says that the hammer was created by the dwarfes Sindri and Brokks and how it's handle came out much shorter than intended due to a mishap in it's manufacture.
Also about the questions about weaponry. The main weapons for the vikings were swords, spears and the axe. Mauls or warhammers did not come into use before Medieval times, where they were used as tools and as improvised weapons when needed.
/u/Gugg256 gives a great answer about how Vikings imagined Thor's hammer to look.
To add to it, here's what actual hammers from a Viking tool chest (found buried in a bog) looked like:
Sketches of the originals: http://home.comcast.net/~meadmaker/images/Hammer1.gif
And a reproduction: http://fc01.deviantart.net/fs17/i/2007/199/9/4/Mastermyr_Hammer_by_hawkthrower.jpg
You'll notice the shape isn't so different from some of the necklaces / artistic versions of Thor's hammer.
