Remember the Panasonic BP2422 solar cell? It was small and tidy, generating a decent voltage suitable for driving low-voltage electronics in a small (24x22mm - less than 1 in^2) package. Alas, they've gone the way of the Dodo, but Solarbotics has gone out and manufactured our own version, in the same footprint, and similar performance using monocrystalline silicon.
Performance Specifications under 300W halogen lamp, at a distance of 6" (our close-approximation to sunlight until next summer solstice):
Open circuit Voltage: 3.4V
Short-circuit current: 12mA
Maximum power: 17.5mW @ 500ohms
Size: 24x22mm (0.94" x 0.87")
Take a look at the datasheet for full curves.