[cesarb]

> Why do they use two bits for it?

To increase the number of 16-bit instructions. Of the four possible combinations of these two bits, one indicates a 32-bit or longer instruction, while the other three are used for 16-bit instructions.

> Do they plan to support other instruction lengths in the future?

They do. Of the eight possible combinations for the next three bits after these two, one of them indicates that the instruction is longer than 32 bits. But processors which do not know any instruction longer than 32 bits do not need to care about that; these longer instructions can be naturally treated as if they were an unknown 32-bit instruction.