The owner of the shop emailed me back, here's what he had to say.

"Good fit is a big consideration. The Biria only comes in two sizes thus contributing to it's affordability. The other size is a 55 and may be a bit too large. You're welcome to try a 55, but if you're comfortable now, I believe the 55 will be harder to handle. I personally prefer smaller bike fit in the city, much more agile. On long rides, a larger bike has it's benefits. Ride on"