The general guideline between tier 1s is to filter at the /24 and shorter level. (/23, /22, etc..)
Longer prefixes between peers will usually be dependent larger aggregate blocks to pass between peers. Since PI space is doled out at /24 and larger, it usually doesn't encounter filtering based on mask length. I believe there are a couple filtering at /21 level still. (not 100% on that)
Glen, the nsp's don't really care if it is PA or PI space and don't compile lists like that.
So if i had my own addresses that couldn't be agregated, say of prefix length /23 i could well be stuffed due to the filtering that could occur between tier 1's
I say this as we qualify for our own /23 small pa space with RIPE. If we chose to go down that route and announce our routes with bgp via 2 upstream isp's through multihoming we could get filtered.
It sounds like we may be better getting address space allocated out of one isp's aggregatable block and announce this over both isp's so that way at least we could rely on the shorter prefix aggregates getting announced everywhere.
With a /23 you should be fine since /23 > /24. With your PI /23 and 2 upstreams, you should have no problems announcing your route globally. I personally think it is better to get PI space than PA if you qualify for it. If you change upstreams later on and have PA space, think of the nasty renumbering job you have in front of you.
When it comes to address space, think in terms of 32 bit addresses, so it helps to be able to think in binary (well, kinda).
larger (more total ip addresses) netblocks have shorter netmasks. smaller netblocks have longer netmasks.