• 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 biomolecules. Using the coordinates of the structures of RNA in the Protein Data Bank we have characterized their sizes, shapes, and flexibilities. The size of the folded RNA structures is measured in terms of the radius of gyration, RG, which is found to obey the Flory scaling law, namely, Rg = 5.5 N^(1/3) A. From this perspective folded RNA structures are as compact as globular proteins for which we obtain Rg = 3.1 N^(1/3) A. The shape of RNA molecules is characterized by the asphericity parameter and the shape parameter S that are computed using the eigenvalues of the moment of inertia tensor. For spherical molecules the asphericity paremeter is zero. If S is greater (lesser) than zero then the molecule is prolate (oblate). From the distribution of ,sphericity parameters we find that a large fraction of folded RNA structures are aspherical and the distribution of S values shows that RNA molecules are prolate. Thus, although Rg shows that RNA molecules are compact their shapes are aspherical and prolate. The flexibility of folded structures is characterized by the persistence length lp which is obtained from the distance distribution function P(r) that is computed using the coordinates of the folded RNA and protein structures. Surprisingly, we find that for RNA and proteins P(r) functions can be fit using the worm-like chain model (WLC). By fitting the calculated P(r) functions to an analytic expression for the end-to-end distribution functions for r=Rg > 1 we obtain lp for RNA. We find that lp ~ 1.47 N^(0.36) A. This counterintuitive result is rationalized in terms of entropy loss upon folding. A simple model of unfolded chains, that is appropriate for RNA at low ionic concentrations, shows that lp is independent of N. By comparing shapes of proteins and RNA we surmise that tertiary folds of RNA are not as densely packed as proteins. The presence of substantial number (compared to RNA) of side chain-backbone and backbone-backbone contacts makes the interior of proteins dense. As an illustration we also analyze packing in the structures of ribosomes (30S, 50S, and 70S) in terms of Rg, asphericity, S, and lp. The 70S and the 50S subunit are highly 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.