  Determination of sizes and flexibilities of RNA molecules is important in
understanding the nature of packing in folded structures and in elucidating
interactions between RNA and DNA or proteins. Using the coordinates of the
structures of RNA in the Protein Data Bank we find that the size of the folded
RNA structures, measured using the radius of gyration, $R_G$, follows the Flory
scaling law, namely, $R_G =5.5 N^{1/3}$ \AA where N is the number of
nucleotides. The shape of RNA molecules is characterized by the asphericity
$\Delta$ and the shape $S$ parameters that are computed using the eigenvalues
of the moment of inertia tensor. From the distribution of $\Delta$, we find
that a large fraction of folded RNA structures are aspherical and the
distribution of $S$ values shows that RNA molecules are prolate ($S>0$). The
flexibility of folded structures is characterized by the persistence length
$l_p$. By fitting the distance distribution function $P(r)$ to the worm-like
chain model we extracted the persistence length $l_p$. We find that $l_p\approx
1.5 N^{0.33}$ \AA. The dependence of $l_p$ on $N$ implies the average length of
helices should increases as the size of RNA grows. We also analyze packing in
the structures of ribosomes (30S, 50S, and 70S) in terms of $R_G$, $\Delta$,
$S$, and $l_p$. The 70S and the 50S subunits are more spherical compared to
most RNA molecules. The globularity in 50S is due to the presence of an
unusually large number (compared to 30S subunit) of small helices that are
stitched together by bulges and loops. Comparison of the shapes of the intact
70S ribosome and the constituent particles suggests that folding of the
individual molecules might occur prior to assembly.
