I meant if you're trying to *implement* serialisation. The Bits
   class allows you to access bits one by one, but surely you'd want
   some way to know how many bits you need to keep?

I think that falls into the realm of protocol design; if you're doing it
in your program at runtime, you're probably doing it wrong.  (The fixed
size version makes sense for marshaling; it's *dynamic* sizes that need
to be thought out beforehand.)

If you're doing, say, cryptography, then thousand-bit random integers that need to be serialised are fairly common...