depends what you mean buy largest.What is the largest currently available rocket kit that can be flown on an Estes D12-3 ? I have a LOC Graduator and that may be it. I would say around 13 oz. is max..
Not to mention it's an upscale mosquito. That basically makes it a 10 out of 10.The Madcow Mozzie is pretty close to the limit for a D12-3. It's between 10 and 11 ounces dry weight, and hits about 280 feet on a D12. It can take up to a G80 stock, or even a 29/180 case if you add enough nose weight. It's got a 29mm mount, but a 24/29 adapter is real easy to make.
For the D12, which has an average thrust of 10.7N IIRC, the theoretical max liftoff weight is 7.7 oz. However, it's got a high initial spike of over 20N, so up to 13 oz works with stable rockets in calm air.
2 more that I would like to suggest that are fairly large, the cfx six footer and the quesar one sky needleThe Mercury Engineering Grave Danger is a 3" rocket with a 29mm mount that can be launched (barely) on a D12. It has a nominal weight of 11 oz. unloaded, and according to the website on a D12-3 it will go up 240 feet. It does much better on composite 24mm motors - 800 feet on an E15, according to Mercury. The company lists the G80 as a recommended motor, too, which they say will boost it up to 2,000 ft. The Mutant Daddy and the Invader are also 3" kits that have 24mm mounts, weigh 8.5 oz. empty, and will go up to 300 feet on a D12-3.
If you are up for clustering, the Mercury Eng. RDR-II is a 4" rocket that takes a pair of 24mm motors. It will get to 400 feet on a pair of D12-5s, according to Mercury.
An Estes Executioner is a nice big rocket that, with a claimed empty weight of 8.1 oz. when built stock, can fly on a D12. To make it launchable on a G80, you would need to replace the 24mm motor tube with a 29mm one, replace the centering rings, and trim the tabs on the TTW fins (or just make new fins). This will increase the weight a little bit, which will harm the performance on a D12. None of this would be difficult for a builder with a little bit of experience. The tube and the rings are available from a variety of sources online.
I suspect that the problem with finding a large MP rocket that can be flown on such a broad range of motors is due to including the D12 in the range. Raising the bottom end of your impulse range just a little bit (to, say, a composite E such as the E15 or the soon-to-be-released E20) would make available a wide selection of kits that will perform well with that entire range of motors.
But if you want a very large rocket that can be launched on the motors at the low end of your range, you could always build this kit.
Aerotech Cheetah is D-G engines...I am looking for more of a sport rocket. Something I could fly on a Estes D12-3 up to a G80. I am looking for length and width in the rocket. That is why I thought the LOC Graduator may be it or there might be a larger slightly lighter weight rocket I don't know about. Thanks !