Possibly worth a look at the PHD Minim 300 in the PHD Sale which starts tomorrow (preview available now). A 3 season bag which PHD rate to zero, but I've used mine a few degrees below this - a lot depends on whether you sleep warm or cold of course*.
The basic model is a simple box wall design with no zip. They offer a short version (I assume they will in the sale) and at 5' 6" you're right on the limit for it. I'm 6', which is the upper limit for their standard size and I find that fine, so hopefully the short will be the equivalent for you.
( * if it ended up not quite warm enough you could probably get PHD to add a bit more down later - not sure what this would cost though )
** or go for a Minim 400 (-5C rated) in the first place, but that's £250 and not in the sale.
Edited: 19/09/2011 at 22:01