[dheera]

26^5 is 11881376, surely there aren't that many users here yet (?)

[OJFord]

And 36^5 is even a bit bigger. (Numbers are allowed. Think that's it though - case is recorded, but I don't think you can sign up Dheera, because dheera exists.)

[mkl]

Underscores and hyphens are allowed too, so it's at least 38^5 ≈ 79e6.