I dunno how small you want it, but I use a Softie seat back chute with aerobatic harness and it's only about 2 inches thick at the rear.  One caveat though is that my plane was built to accommodate large people, and I am 5'10" and need a couple of inches of cushion behind me anyway, and the seatback parachute works perfectly.  I don't wear it often so it probably won't be there if I ever need it.  I'm not quite sure I could egress properly from the Glasair, but you can bet your behind if I was wearing it and needed to get out I would give it a shot.  I think these planes would be difficult to bail out from.  Kinda gives you the willies knowing you don't have a reserve chute either :-p.  I would guess by the time we got out of these planes at the altitudes we normally fly we wouldn't get a shot to cut away and pull a reserve anyway.

Steve Reeves
Glasair 343ND
(Hoping my chute never gets used)