Size of what? The tabletop? My old Dewalt is a 2 pce setup; side to side dimension is irrelevant on mine but the front to back is a bit tricky because the fence board gets clamped between the two table pieces. There are a couple of threaded clamping fixtures that determine the total front to back measurement.
Looks like yours is similar.
If so, adjust the clamps to the middle of their range, measure the front to back clamp distance, deduct the thickness of the fence strip, and that's your total table top front to back dimension. where the fence goes is determined by making sure the blade is completely behind the front of the fence when it's fully retracted to the rear, and the blade bottom is at tabletop surface ht. For ripping plywood, you can put the fence at the back of the back top panel...that should give you around 24" of rip width with the blade swung around to the outside of the yoke position.
It goes without saying (but I'll say it anyway!) ...make sure all the anti-kickback protection is fully functional and deployed when ripping on a Radial arm saw!!! Bad sh*t happens very fast.