Whats the width of a standard A2 front sight?  I'm wanting to replace my stock A2 sight post with a National Match (.050", .040" or .030") but would like to know how wide a standard sight is for reference.

I'm going to assume that you plan to shoot matches with this...

The standard front sight post is .062", but consider this before switching to something smaller:

Smaller front posts are more stressfull to concentrate on when shooting across the course.  If you find your eyes give out by the time you get through the prone stage, think about switching back to a larger size.  The same thing goes for the rear aperture BTW.  I've never seen anyone on the line with a post smaller than .050, and a lot of competitors use standard width .062" posts that have been cleaned up.

In summary:  Go ahead a try a smaller post, but keep in mind that the first few groups are not going to indicate how it will perform for you across the entire course of fire.  If it's better, great, but there's only one way to determine that...

FWIW, I use a .052" front post for matches.

On a 20" Bushmaster, 0.07" here.

Front posts on Colt carbines may be 0.062" to give a more rifle-like sight picture. A1 post measures around .065" at the tip.