Based off of "Up to 20 charges from a D cell" I'm going to guesstimate that it takes ~500mAh to fully charge.
On another site, I found that it takes 2-4 hours to fully charge. So you'd need 125-250mA @ 1.5 volts. That's about a 3x4" panel.
The cheapo multi voltage harbor freight panels ($10) allegedly do 3v @ 200mA. In practice, charging one nimh, I get about 120mA sitting in my window, although this might not drop in voltage enough to be safe for the Rex.
I'm not sure if powerfilm makes panels smaller than 3 or 3.6 volts, but they do make quite an array of sizes, and for something this small, you should be able to find one for under $20. They're super thin and flexible, although not all of them have a UV protective coating on them, so make sure if you do order.