Some works of art cry out for large canvases. Though it is sparing in its use of scenery or anything approaching spectacle, J. T. Rogers's 'Oslo,' an against-the-odds story of international peacemaking, is undeniably a big play, as expansive and ambitious as any in recent Broadway history. So it is particularly gratifying to announce that it has been allowed to stretch to its full height in the thrilling production that opened on Thursday night, directed with a master's hand by Bartlett Sher.