Permanent magnets have a saturation magnetization. Trying to apply stronger fields doesn't magnetize the material more, and if you drop the external field then the field of the magnet decreases, too. In practice you get around 1.3 T for neodymium magnets, theoretical values might be slightly higher. This publication calculates 1.32 to 1.38 T.

The size of the magnet doesn't matter, you just scale up everything linearly in space and the field gets larger but not stronger.