design of folded cascode
As Raduga suggested, start with biasing currents, 70% seems like a good idea. Then, think how much overdrive you want to allocate to specific transistors. Keep in mind that your output swing is directly determined with allocated overdrives. Then, you select transistor dimensions, so that these transistors can provide the necessary current for the chosen overdrive voltage. by choosing greater L you'll get more gain, but you may get problems with a low pole, as well. if you need single ended output and have a problem obtaining enough swing, try with high-swing cascode configuration.