I thought that the micrometer I was using to move the razor through the laser beam was metric; however, it is actually english.
After discovering this mistake, I converted my previous measurements to centimeters and fit the data to
w = sqrt(w0^2+lambda^2*(z-z0)^2/(pi*w0)^2) with the following results:
reduced chi squared = 14.94
z0 = (-4.2 ± 1.9) cm
w0 = (0.013 ± 0.001) cm