Can I define 21-bit long instructions with TableGen?

I am a newbie working on a VLIW ISA. It encodes three 21-bit long individual ops, plus 1 bit specifying if the ops in bundle execute in parallel. The whole bundle has a fixed width of 64-bit (21 x 3 + 1 = 64).

So my question is, can I define these 21-bit instructions in those td files? I looked into other targets, they have a size field, expressed in number of bytes, not bits. For example, a 32-bit long instruction has a let size = 4; in those td files.

The “size” of an instruction, as set in the instruction description, isn’t actually used by any target-independent code, as far as I know. So you can use the field to mean anything you want. Most targets set it in bytes because they use it to implement getInstSizeInBytes().