Here is a list of every starting center in the league with their corresponding height and weight. Data collected from ESPN.com roster section for each team:

Perkins: 6'10 280
Collins: 7'0 260
Curry: 6'11 285
Dalembert: 6'11 250
Bosh: 6'10 230
Biedrins: 6'11 245
Kaman: 6'11 245
Bynum: 7'0 275
Stoudemire: 6'10 245
Miller: 7'0 261
Wallace: 6'9 245
Illguaskas: 7'3 260
Wallace: 6'10 230
O'neal: 6'11 260
Bogut: 6'11 245
Chandler: 6'11 245
Dampier: 6'11 265
Ming: 7'6 310
Duncan: 6'11 260
Milicic: 7'0 265
Horford: 6'10 245
Mohamed: 6'10 245
Shaq: 7'0 340
Howard: 6'11 265
Haywood: 7'0 263
Camby: 6'11 245
Jefferson: 6'10 256
Pryzbilla: 7'1 255
Thomas: 6'9 245
Okur: 6'11 263

The listed statistics for Amir Johnson from th same source are as follows:

Johnson: 6'9 210

Granted, the height and weight may be a little low right now, but clearly Amir is NOT BIG enough to be a 5 at his current size. Even Amare who Amir is most compared to outweighs Amir by 30-40 pounds. What is Amare's biggest weakness ( a weakness many blame on the Sun's never being able to advance farther in the playoffs) ? Post defense. Basically, Amir cannot even be considered a center until he puts on much more size. Taking a cursory look at the 4's in the league, Amir is still quite light in the pants.

On a side note, take a look at some of the SF's in the East:

Pierce: 6'9 230
Prince: 6-9 215
Deng: 6-9 220
Butler: 6'7 240

Clearly, Amir is the absolute correct size to be a 3 or a 4. Neither too small or big for either. Time growing and learning a particular skill set may push him toward a specific positions, but using his size to exclude him from these two positions is completely farce.