This article is from the Model Rockets FAQ, by Wolfram von Kiparski with numerous contributions by others.
Large Model Rocket (LMR) is an FAA designation for a model rocket that is
between 454 grams (16 ounces) and 1500 grams of mass, including propellant,
-OR- contains more than 113 grams (4 ounces) but less than 125 grams of
propellant (all motors).
Please note that this definition is different from the definitions of a
model rocket found in NFPA 1122 and 1127-94. Some rockets may be defined
as LMR by the FAA, but are actually HPR according to the NFPA.
If you are planning to fly a Large Model Rocket as defined by the FAA, you
must notify the "FAA ATC facility nearest to the place of intended
operation" 24-48 hours before launch.
FAA notification requirements appear in FAR 101, and all of the pertinent
regulations appear in Part 10 of this FAQ.
A sample FAA notification form is available at the sunsite archive at:
From Bob Kaplow (email@example.com)
"you need to find the right FAA office to notify. If you don't know
where they are, call 1-800-WX-BRIEF, and ask a briefer for the
proper place to call. Get an FAA map of your area. From that you can get
latitude/longitude, and radial/distance from your launch site to a nearby
VOR. Call 24-48 hours before the launch and give them the information.
I usually write a letter and mail/fax it to the folks first. That way
they have everything in writing. As a courtesy, I also call the closest
flight service station (FSS - ask the folks at 1-800-WX-BRIEF where they
are) and ask them to issue a locak NOTAM. Get the "L" number as proof
that you called."