That is why Dobermans deemed as "giant" "large" "king" or "warlock" are discussed in bad terms by those that know and truly love the breed. Not only does the larger size go against the standard set forth by the parent breed club but the larger a dog gets the more strain on his joints, tendons, muscles, bones, heart, etc. As Gelcoater said the larger they get the harder time they have moving as they should and performing the job they were originally created for. The standard is set as it is because that is the size, look, and temperament that is ideal for the breed. When you deviate too far from that standard you run into problems.
Please please please do not support pet stores like ones found in malls. The "purebred" dogs they have there come from puppy mills. By supporting them you are supporting those that run puppy mills and the terrible conditions in which they breed. If you want a puppy do your research on the breed you are interested in and if you find they will be a good fit for your lifestyel find a responsible breeder that breeds for health, conformation, temperament, and for the betterment of the breed.