See section 1.5 ("Base Instruction-Length Encoding") of the RISC-V spec. It's actually a bit more complex than just using 2 bits (I had forgotten those details)

See section 1.5 ("Base Instruction-Length Encoding") of the RISC-V spec. It's actually a bit more complex than just using 2 bits (I had forgotten those details), but the basic idea is the same in that there is a fixed cascade of bits identifying the instruction length. There aren't any standard extensions with instructions >32b yet, but the extensibility is there in the base spec.