Generally, a /24 is the smallest block for which most networks will accept BGP advertisements. You'd generally purchase a /22-/20 or larger though. Note that in

Generally, a /24 is the smallest block for which most networks will accept BGP advertisements. You'd generally purchase a /22-/20 or larger though. Note that in most cases, these are (part of) blocks that were assigned before the current regional registries existed, as blocks allocated through the registries aren't actually owned.