Wikipedia puts the LD50 for inhaled or injected ricin at 22 micrograms per kilogram in humans; with ingestion considerably less dangerous.
Ricin + explosives sounds like either an amateur who is hoping that brute force will handle aerosolizing the payload adequately; or a (probably more sensible) plan to build a basic small bomb with shrapnel and use the shrapnel to inject the toxin, as well as significantly complicate safe removal of the shrapnel.
There’s no substitute for size when it comes to explosions; but toxic shrapnel could make a real difference for everyone far enough away to only catch a piece or two, rather than the full force of the blast, and at best make treatment significantly more difficult.
