46mm for the rear wheel. Go to Halfords and buy a 1 13/16" ball-joint socket. But most of all you'll need a foook-off great breaker bar! And 41mm for the other side (sprocket side).
Someone will be along shortly to tel you the front size. I can't remember for sure, I think it's 28mm. It's a bit of an odd size, anyway.
Yes, 28mm for the front which is a very odd size. I kept an eye out whenever going past DIY type places and managed to pick one up for 3 quid. Same with the 41mm and 46mm - 7 quid each from a local car place.
Don't forget you will need to unbolt the front calipers to remove the wheel, I think they are 8mm (or maybe 10mm) allen type & best to use socket type instead of key so you can torque correctly.
Be carefull when tightening front wheel nut as Antonye discovered this will split in half if overtightened.
As Martin says, the 28mm nut is only used to draw the axle through the fork legs into the right place. The fork pinch bolts (the smaller ones) are then used to hold the axle tight. So don't overtighten it!
Have just invested in a set of shiny titanium ones from Casoli, and need some new tyres, so double the reason to get the bolts off!
Currently got some of the lads turning the workshop and Bodyshop over to see if we have any of these sockets, 'cus otherwise my £90 titanium nuts are going to get a little more expensive!