While the smallest cache size is 256MB, larger is better. Sizes larger than RAM can be beneficial, so 1:1 for RAM sizing is not a useful rule of thumb for sizing.
ReadyBoost requires 2.5MB/sec for reads and 1.75MB/sec for writes. Typical USB setup is exFAT with 4k sizing.
ReadyBoost was introduced in Windows Vista for one flash drive. The flash drive capacity was increased in Windows 7. Windows 8.1 allows for multiple ReadyBoost drives.
After ReadyBoost is running, it can be monitored with Perfomance Monitor. Press Windows+X, choose Run, type perfmon
Press the red X to stop monitoring.
Press the green + to configure.
Click ReadyBoost Cache
Press OK and monitoring will begin.