operand size
How many bits an instruction works on - 8, 16, 32 or 64 - chosen by the register names it uses or by a size word.
The operand size of an instruction is the width of the data it reads and writes; x86-64 selects it with the register name or, for memory, a size keyword, ARM64 with x versus w registers and the load/store mnemonic, RISC-V with the mnemonic (ld/lw/lh/lb, addw).
See also