Nitpicking, but the documented RISC-V encoding supports anything from 16-bit through 176-bit long instruction words in 16-bit increments, with instruction length being determined by the first 16 bits of the instruction word. Some encoding space has been reserved for 192-bits or longer. Recent versions of the RISC-V spec have actually de-emphasized this length encoding, so implementations may well be allowed to use the encoding space in different ways as long as they don't conflict with existing 32-bit (or 16-bit if the C extension is specified) instructions.