Not sure, but it seems to me that for a mirror, more than albedo matter, because mirrors have a specular reflection rather than a diffuse reflection.

Now, reflection won't be perfectly coliminated, but if you assume it is, then the mirror essentially becomes a piece of sun. Since the sun is about 400 000 times as bright as the moon, you'd need to scale the diameter of the sun by sqrt(400 000 / 8) =~ 223. Applying that scaling to your 5.4Km yields about 24m of diameter, which seems waaaay to small to me, so I am probably wrong.

That feels really