Instruction Size

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.

[goggles99]

Just to add one more that I have found (and actually used). It's written in C++
it's called InstLenDisasm, you can find it here.
http://www.security.org.sg/code/antihookexec.html <-- interesting article too btw
look inside the antihookexec zip file.

also, since z0mbie's website seem to be gone, you can get ADE32, LDE, and XDE
here
http://www.madchat.org/vxdevl/engines/
or here
http://vx.netlux.org/vx.php?id=eidx&page=0