  We determine effective collisional radii of positronium (Ps) by considering
Ps states in hard-wall spherical cavities. $B$-spline basis sets of electron
and positron states inside the cavity are used to construct the states of Ps.
Accurate Ps energy eigenvalues are obtained by extrapolation with respect to
the numbers of partial waves and radial states included in the bases.
Comparison of the extrapolated energies with those of a pointlike particle
provides values of the effective radius $\rho_{nl}$ of Ps($nl$) in collisions
with a hard wall. We show that for $1s$, $2s$, and $2p$ states of Ps, the
effective radius decreases with the increasing Ps center-of-mass momentum, and
find $\rho_{1s}=1.65$ a.u., $\rho_{2s}=7.00$ a.u., and $\rho_{2p}=5.35$ a.u. in
the zero-momentum limit.
