Just trying to get some clarity and direction regarding
obtaining address space/ASN for my client.
Is there a minimum address space (?) an entity would need to
justify to go directly to RIR (ARIN in this case) as opposed
to the upstream provider? Is /20 the minimum allocation? Can
my client approach RIR and request for a /23?
The minimum assignment from ARIN is actually a /22, which will only be
given to multi-homed end-users who qualify
(http://www.arin.net/policy/nrpm.html#four3). Other than that, the
smallest assignment is /20.
If my client do procure a /23 how do they make make sure that
this address space will be globally routable?
Regardless of where their IP's come from, all they have to do is
announce them to their upstream providers using BGP.
Multihome will also be part of their network implementation,
can they apply for an ASN number?
Yes, they can apply for an ASN number as long as they can show ARIN that
they are in the process of bringing up connectivity with more than one
ARIN's web page is actually pretty easy to navigate for finding this and
all the other information you will need. I would suggest checking it