Well, I may be jumping the gun a little since I have an S4 and I don't know if there's a difference between that and your 900. On the S4, you have to pull the brake calipers first. Just undo the allen bolts and pop them back and to the side. Dunno off the top of my head what the nut size is, if I'd have to hazard a guess I'd say it's a 28 but don't quote me on that one.
On the S4, you need a key that locks into the axle. Looks like a tube with a t-fitting. $3.- from my Duc dealer. It locks the axle to a little grooce in the right side fork to keep if from turning when you tighten/ untighten the nut. With that key in place, undo the nut, slide out the axle, and the front wheel should pop right out.
Oh, I think I have to undo the fender mount bolts as well, you may not have to do that on yours.
On my 900 95 Duc it's an M14 Hex drive on the Brake side to extract the front axle. loosen the two small Socket Head Cap Screws the ones at the front on the bottom of the fork (Brake side)
just make sure you don't loosen the two small Socket Head Cap Screws on the Clutch side of the axle shaft (the ones at the front on the bottom of the fork) otherwise you'll shear off your Speedo drive locating pins, like i did a week ago.