Minimum globally routable address space?

["Dan Streufert" <dans () icss com>]

I am writing from a small, multi-homed ISP.  We are contemplating
terminating our relationship with one of our providers, and will need to
replace that non-portable space.
We are debating between getting a /20 from ARIN (we have justifiable usage)
or a /20 from our other provider.

On one hand, portability would be nice.  On the other, is a /20 globally
routable in practice? I seem to remember hearing that /19 was the smallest
that would not be filtered by anyone.

If anyone knows if there are filters for /20s out there, I would be
interested to know who filters them.

                Thanks,
                Dan Streufert