There have been very few images that show Centurions alongside other models with known base sizes. In fact, the only one I've been able to find is this image of the Lastrati Crusade collection:
|One of the few pictures that allow a comparison|
Notice that the Centurions are taller than the nearby Assault Marines (one upraised arm is approximately level with the top of a nearby Marine's jump pack) but their bases are smaller than those of the Dreadnoughts behind them. With a little help from Photoshop, I was able to compare the Assault Marines' 25mm bases to a Centurion base:
|Centurion bases turn out to be two 25mm bases wide|
All of a sudden, the cause of the early confusion becomes clear. Centurions aren't on 60mm or 40mm bases; they're on 50mm bases.
Since there aren't any 40mm bases in the images above with which to make a comparison, I did some additional analysis to confirm that Centurions aren't on Terminator-size bases. Using Photoshop's grid feature and calculating the appropriate ratios, I was able to make a comparison with an Assault Marine:
|50mm Centurion on the far left, 40mm Centurion on the far right|
The Centurion on the left is scaled as if it has a 50mm base. The Centurion on the right is scaled as if it has a 40mm base. While the 50mm Centurion is noticeably taller than the Assault Marine (the upraised arm is nearly level with the jump pack, just as it is in the Lastrati Crusade image), the 40mm centurion is approximately the same height. Since we know that Centurions are taller than Assault Marines based on the Lastrati image, the larger of the two base sizes must be correct.
Knowing that the Centurion is on a 50mm base, I was able to compare the sizes of several common Astartes models:
Although slightly smaller than a Dreadnought, Centurions are still relatively large models.