Whenever possible, I use the rip fence as a guide to crosscut wide panels. Although a half sheet of plywood can be a bit unwieldy, the end of the panel riding against the rip fence will provide a stable guide edge. The trick is to keep the end snug to the fence while feeding the panel.
A simple hold-down will help control large panels of plywood on the table saw. Just clamp a long cleat to the rip fence to trap the panel. You’ll need to push the trapped piece completely through the blade, so position yourself with one hand on each side of the blade to finish the cut.