Hi Seth, and Mark
A mastiff/Doberman mix is bound to be a VERY big dog. Mastiffs are known for their large size (English Mastiffs are the heaviest dogs on record, from what I remember, and any breed of Mastiff is going to be a big, big dog when he/she grows up). Dobermans, though generally slimmer and less gigantic than Mastiffs, are also quite large, so the combo of the two means you will still be looking at a big dog. I had a Mastiff named Jake when I was a kid, and he was a gorgeous, gigantic boy!
Also, both Mastiff and Doberman are "guard" type dogs. Many people get them as protectors, because they are large and in-charge. However, that doesn't mean that they can't make great family dogs, especially if you are raising them from puppyhood. They might require extra training because you need to be extra sure that they will not end up aggressive (their large size makes them more dangerous if they were to lash out at someone, though they aren't necessarily going to be predisposed to aggression).
Do you have any experience with dogs? If not, this breed mix might be a bit challenging for you. However, I would recommend that you go to your local library and check out some books about positive reinforcement dog-training (most dog-training books I've come across are based on a system of positive reinforcement, where you ignore or stop bad behavior, and reward good behavoior). It helped me LOADS with my dog, and is good whatever breed you might choose.
You might also want to look for some dog breed books. Though there likely won't be any books on your specific cross (Mastiff/Dobe), there will likely be books about each individual breed, which will give you some good indication of what to expect.
P.S.- Another tip I learned through experience: whatever dog you bring home, give yourself a few weeks to adjust. When I first adopted a dog, I started freaking out because things didn't seem to be working right away. I thought I'd never be able to adjust to my new responsibility, and my new dog was acting crazy, hyper, unmanageable. Once I started training her and began relaxing back into my normal life with her around, she calmed down and began to learn the ways of the apartment, and now she fits in wonderfully!