reasonable average persistence length

Reasonable values range from about 3 nt to maybe 15 nt. It is possible that the length might be longer than 15 nt, it is very doubtful that RNA is every much less than 3 nt. The current lower limit is 3 nt for this reason. There is no official upper limit aside from perhaps $\xi < N/3$, where $N$ is the total length of the sequence.