Maybe I'm getting old and feeble, but wrestling a 4*8 sheet of plywood thru the radial arm saw single handed no longer feels safe. I fear the sheet will wedge the blade and cause a kickback, or the unsupported cutoff piece will fall to the floor, tearing the material , or I won't be able to keep it going straight.
So for many years I have been dissecting the big sheets by laying them on saw horses and having at them with a hand held circular saw ("skilsaw" we call 'em up here). Being hand held, getting a straight cut can be difficult. I used to clamp a straight board to the plywood and use it as a saw guide, a trick I picked up from an R. J. DeChristoforo book. As long as the board was truly straight (not guaranteed with modern lumber) and stayed pressed tightly to the plywood, that works.
Yesterday I came upon a better method. For rip cuts the long (8 foot) way, I snapped a chalk line right onto the plywood and then ran the saw free hand following the chalk. This gave me a noticeably straighter line than the guide board gave me.