Assuming that the muscular build of the ostrich did not have to change in order to fly, therefore weight remains constant, the weight of an ostrich is 150kg, and under the caveat that the dataset might not be the best in the world:

I did a small model with a stats program and using a simple linear regression analysis, it would appear that the ratio of wingspan (cm) to weight (g) is 108.14 + 0.011*weight (This gives an R value of 0.72, which considering, isn't too bad!). Extrapolating this regression gives the wingspan required of an ostrich to be 108.14 + (0.011*150000) = 1758.14 cm or 17.5 metres or nearly 60ft.

KFO anyone?