OpenGL doesn't have a "maximum" texture size, but video cards do. Most video cards support textures larger than 256x256, but 3dfx cards (up through the Voodoo3) have had a maximum texture size of 256x256. I'd suggest breaking your title screen into a few different textures and just render them all together.
BTW, you can call glGetIntegerv () to get the maximum texture size of your OpenGL implementation.