GW still sells flying stands in the supplies section. Oddly enough, they FINALLY now sell the small bases with small flight stands (no, I'm totally NOT resentful for buying a big bagfull years ago that I've barely gone through...), although for some crazy reason they sell the large bases with the long flight stands. Anyhow.
If you're looking for exact sizes:
Small Flying Base: 1.25" diameter
Large Flying Base: 6cm diameter
I'm pretty sure the large bases were done in metric, the smaller stands done in inches. If you try measuring the smaller ones across their diameter in cm, you get a really ugly number, same goes for doing the larger ones in inches. Yay for consistency! Go figure.