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

Effective radius of ground- and excited-state positronium in collisions with hard walls