There is also a wide variety of cropping styles. The current Doberman show crop is long and narrow, with not nearly as much bell as the older crops (which were also wider and shorter) Other dog breeds have different styles too, long and narrow like a dobe show crop, all of the way down to the very short, by comparison almost non-existent ears of a breed like the Cane Corso or the Dogo Argentino. If someone has some better pictures as illustrations, help me out!
Dobe show crop
More old-style classic Dobe crop
Great Dane crop
Min Pin crop
Staffordshire Terrier crop
Cane Corso Crop
Dogo Argentino Crop
It sounds like you might be looking at a mixed-breed dobe and trying to figure out what he might look like cropped?? Seems to me that the dog breeds with wider heads tend to have shorter, wider at the base and more triangular crops--the dogs with longer more narrow heads by comparison have narrower and longer crops. Generally.
Cropping is really an art form and not many vets do it really well. A good cropping vet will look at your dog's head shape and try to crop the ear to a length and width that will really compliment his head.
A doberman puppy should be cropped ideally at about 7-9 weeks old. You can crop up until the puppy is maybe 12 weeks old, but that is pushing it. Generally you run more risk that the dog's ears will not stand properly if you wait until after 10 weeks or so to crop. Sometimes to counteract that, a crop can be shorter or wider in an older puppy (up to 12 weeks, max) to make it more likely they will stand.