There is a ton of those on the net.
Bengaly (PVDasm) offers one, there's one on the ollydbg website, z0mbie wrote a few (google for ADE32, LDE - his website seems down). If all you need is the length of the instruction, then the simplest solution is probably ADE32 or as, taipan wrote, ELiCZ's X86IL.

It's a good exercise to write one yourself; you'll master the instruction format afterwards.