Abstract Conformal invariance at a second-order phase transition yields an exact finite-size scaling theory on a two-dimensional cylindrical geometry. We combine this two-dimensional conformal field theory with a numerical investigation of the deconfinement phase transition of (2 + 1)-dimensional SU(2) lattice gauge theory. Results of this study and a more general finite-size scaling analysis on square lattices yield excellent agreement with d = 2 Ising critical behavior, as has been conjectured on the basis of universality.