Originally Posted by tatfiend

Do you mean diameter or length? Most standarrd bicycle front hubs have a width of 100mm between the bearing locknuts and a axle diameter at the fork dropouts of 9.5mm as I recall. I will presume you mean axle length. If it is a quick release hub and the axle is a bit too long to allow tightening the quick release enough to hold the wheel in the fork then the ends of the axle can be ground off to allow correct quick release tightening.

Inspect carefully. Is the problem that the axle protrudes from the outside of the fork dropout on one side or both? If only on one side then a bike shop can possibly recenter the axle for correct fit. If both sides then your fork dropouts are thinner than current ones in which case shortening the axle on both sides will be required.

If you truly mean axle diameter then your original wheel axle probably was undersize or had flats ground into it where it fit the fork dropouts. In that case I would have a good LBS look at things and see about widening the dropouts to fit the new axle.