How do processors fetch instructions that are longer than the word length?
In general there is not relation between the processor register size (32-bit for x86, 64-bit for x86_64) and the instruction size. Instructions are fetched by a dedicated hardware unit and the current instruction is stored the instruction register, but this register does not need to (and usually does not) have the same size as the data registers. On x86, this register must be able to hold 15 bytes. Instructions are fetched like everything else by issuing read commands to the memory. In general to speed up things, the CPU has an instruction cache so it will not just fetch one instruction at a time but rather an entire cache line at once.