I saw this on *bay, the seller says that it's a giant MVVS engine. Looks like a GIANT K&B .61

Has anyone ever used one of these?

It's not a K&B. It must be an old MVVS

http://www.mvvs.nl/MVVS/main_bestanden/glow.htm

There is a cm scale that shows it to be 15cm or 5.9 inches long.

Maybe Pe' will know what it is.

It is Likely MVVS judging by the small parts that are fitted.
It must have been before my time.

MVVS released a 25 cc glow engine in the 1970's.

It caused a stir as no-one had ever seen a glow engine in that size.

It remained a curiosity as no aircraft designs were available for it back then. Also, MVVS were still a small state sponsored "eastern Bloc" manufacturer having limited access to Western markets.

I believe what is offered is likely to be that engine.

Pe', that engine appears to be a loop scavenged engine, maybe the owner could look and see if it has a baffled piston.