I know I'm always beating the mixte drum, but if you can't find anything in your size, you might go that route. Old road bikes under 50cm (or even 52cm) are out there but few and far between. I have an old mixte I've built into a single speed, and I'm looking for a second one (my first is built up as a low-geared townie with racks and baskets and albatross bars, and I want a second one for faster riding, with moustache bars and higher gearing and no baskets). And I'm having a hard time finding one big enough for me! My current one is probably equivalent to a 49cm road bike, but with a shorter reach because of the step-through frame, and most of the mixte frames I'm finding are mostly closer to 43cm. (I'm doing rough conversions; the geometry is so different that it's hard to compare.)
I really love mine ... I originally bought it because I sometimes want to ride to work in a skirt, but it's so springy and fun to ride that I am now a mixte fan all around. Every one I've seen has horizontal dropouts, and overall they are perfect for single-speed conversion. I was inspired by these ones on Fixed Gear Gallery:
http://www.fixedgeargallery.com/2007...naMurphy_4.htm
http://www.fixedgeargallery.com/katybeanlarson.htm