Check out the "smartalign" package in NASM: it contains 1-8 byte nop instructions for several different x86 variants: http://repo.or.cz/w/nasm.git/blob/a2c78555770990ed966c414da9...

You can use xxd and objdump to see which all of these translate into. For example, here's an 8-byte nop for x86-64:

  $ echo '0x0f,0x1f,0x84,0x00,0x00,0x00,0x00,0x00' | xxd -r > /tmp/bincode
  $ objdump -M intel -D -b binary -mi386 -Mx86-64 /tmp/bincode
  
  /tmp/bincode:     file format binary
  
  
  Disassembly of section .data:
  
  00000000 <.data>:
     0:   0f 1f 84 00 00 00 00    nop    DWORD PTR [rax+rax*1+0x0]
     7:   00