By C standards that’s perfectly fine. No crazy macros, just plain old bit-twiddling. The and-not combination is a very traditional pattern for bit masking.

As pavlov said, it's readable enough to many (those that understand bitfields).

It's literally current size + (current size/8) + fiddle

( the fiddle is there to prevent a fail to increase for small input (exercise for reader?) )

followed by a mask to ensure a multiple of 8.

The rational would be no floating point ops and all the weirdness that can entail plus the speed of raw shift | addition | mask.

You're correct that it deserves a twice over as the fiddle raises an eyebrow that invites investigation across inputs.