A quick scan of the current Internet routing table will show you that a large number of /24 prefixes exist. However, this is discouraged and while these ones made it through does not mean that yours will too.
It is obviously not enough that your two ISPs support /24 advertisements. If the ISPs beyond them don't they will get dropped.
My suggestion is: start off with a /24 and see how far through the Internet you get. If it gets dropped somewhere along the way, resort to the /21.
the size of the prefix, which will be assigned by the registry to you depends on what you want to do with the IP addresses.
In any case you will get a /24 or larger, because anything smaller will not be announced by some ISPs due to their filtering policies. Saying this and having a look at load sharing/balancing options for the return traffic from the internet to your AS, a /23 or larger would be nice to have. But still, look at the IPv4 PI address request form and you will see, that you have to justify what you are requesting. Depending on the network size you have (Web servers, DMZ, firewalls, mail server, etc.) you might just get a /24.
The /24 will make it, because a normal ISP will respect the registry assignments. And a /24 is still the smallest unit. Anything smaller I wouldn´t even give a try, so do not split your PI addresses into smaller chunks within your BGP announcements to the internet.
Now to go forward with your PI request, I would make a preliminary design of the internet part of your network (where you can use subnetting internally) and try to figure, what amount is needed.
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