Do people match shoot with 16in. AR's or mostly 20's?Just wondering.

More at local matches where power factors are not required and targets typically not out as far  as at the big 3-gun events.For them 18" to 20" are preferred to get more FPS with heavier or not bullets with a flatter trajectory hopefully,which is the goal, among other things.Same as for SPR's to reach out and touch someone with a wallop.

In NRA highpower matches, everyone shoots at least a 20" bbl.  In Service Rifle, it's required, as the rifle must match the external appearance of an M16A2.  You could shoot a carbine, but you'd fall into the Match Rifle class, where you'd be eaten alive.  

In Match Rifles, many of the AR's have 22" or 24" barrels.

I shot a Garand in a match last weekend(VERY FUN!)It was my first time at a real range!Not a home-built one.It was also my first time shooting a Garand,now I have to buy one of them too!

I you are shooting CMP Service rifle, and using an AR15 or M16, the barrel may not exceed 20" in length. Also, if you are using free-floating handguards, they must be OCMP approved, that is they must look like normal M16 handguards externally. this is done with an internal sleeve, and the plastic bits stripped of the heat shield and modified to fit the sleeve. All outwards appearances must be similar to a service rifle, i.e. M16, although the exception is they do not allow flat tops....yet.

They are quite strict about the regulations.
I called and asked if I could put on anti-walk trigger/hammer pins...the kind with the external clips, they said no, becasue it modifies the external appearance.

Do i have to have a standard cary handle(not removable)?

At the moment, only the standard non-removable A2 carry handle is permitted. I called today because I have a friend who wants to compete in service rifle, but he also wants a flat top AR15(with removeable handle), he'll have to give up on one of the two. They said it is still "under review" but has been for some time. Not this year.

You can read the official rulebook here:
[url]http://www.odcmp.com/Competitions.htm#SERVICE%20RIFLE%20AND%20PISTOL%20COMPETITIONS
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If someone is shooting with a 22" or 24 inch barrel with an AR15, they are not shooting service rifle.

That sucks,I just sold my 20in. A2 upper.Is it worth buying a "national match" upper compared to a regular A2 upper half?

Well while you can compete with a standard 20" A2 upper, you wont be competitive (doesn't mean you can't have fun).

Many shooters will use the "DCM" uppers you see for sale. These differ from most standard A2 uppers in several areas. They will usually have 1-8 twist Stainless steel heavy competition profile barrels (1" diameter under the handguard, but .75 milspec HBAR diameter where visible. They will have the 1/4x1/4 or 1/2x1/2 MOA adjustble rear sight assembly with muchsmaller apertures, sometimes hooded. they will have single or 2 stage competition triggers of at least 4.5 lbs combined pull, free floating ODCMP approved handguards, and other touches that aren't visible externally.

I just bought an Armalite DCM upper to compete with. Check out [url]www.compasslake.com[/url] they have very good DCM rifles and components. they also will modify your existing rifle or sell just the upper. not cheap but among the best. I paid $375 for my Armalite upper (used) at a gun show. It has all the features I listed above. the stainless barrel takes some getting used to, but it is better for this type of shooting. Bushmaster anodizes their Stainless barrels black on their DCM rifles.

Again, you can compete with a standard A2 profile, but the sights alone will limit your accuracy. The .36 aperture 1/4 MOA sight will force you to shoot more accurately, all other things being equal.

Bushmaster, Armalite and RRA all make DCM rifles out of the box, as do many other custom manufacturers such as georgia precision, Compass lake, etc.