You'll never be able to drive that gantry with one screw. It'll skew a lot. You can probably put a screw on each side, and still keep them under the table.
If you plan on cutting 4x8 sheets, you'll need 9 or 10 ft screws. Most DIY machines that size use rack and pinion to avoid the long screws. If you want to use screws, you'll probably need to use 1" dia.
How fast should you go? as fast as possible, or as fast as your budget permits. You're cutting speed will be limited by your router, but you can always take multiple passes as deep as the router will allow.
I'd shoot for at least 200ipm, preferably more. Here's some info on choosing motors courtesy of GeckoDrive.
Q.) Should I use servos or steppers in my machine?
A.) If you are designing a machine and you get to motors, the first thing you should do is calculate the power you need. Never buy a motor (stepper or servo) first and then figure out if it will fit what you need.
Motors are motors. They couple power to your mechanism and power is what makes things happen. The choice of a motor comes after you know what's needed.
Power is velocity times force or torque times RPM. It doesn't matter if the motors are steppers, servos or a gerbil in a spinning squirrel cage at the start.
To separate what motor need (neglect the gerbil), is the power your mechanism needs.
Rule #1: If you need 100 Watts or less, use a step motor. If you need 200 Watts or more, you must use a servo. In between, either will do.
So, how do you figure the power you need?
Method 1: You have a plasma table, wood router or some other low work-load mechanism. You have a clear idea of how many IPM you want but you’re not sure of what force you want at that speed.
Pick the weight of the heaviest item you are pushing around. If it weighs 40lbs, use 40lbs. multiply it by the IPM you want. Say that's 1,000 IPM. Divide the result by the magic number "531". The answer is 75.3 Watts so use a step motor.
Eq: Watts = IPM * Lbs / 531
Method 2: You have a Bridgeport CNC conversion you are doing. The machine has a 5 TPI screw and you need a work feed rate of 120 IPM. 120 IPM on a 5TPI screw 5 * 120 or 600 RPM.
How about force? Not a clue? Use your machinist's experience on a manual machine. The hand crank is about 6" inches in diameter. How much force would you place on the hand crank before you figure you're not doing something right? I hear about 10 Lbs.
10 Lbs is 160 oz, 160 oz on the end of a 3" moment-arm (6" diameter, remember?) is 480 in-oz (3 times 160) of torque on the leadscrew.
The equation for rotary power is: Watts = in-oz * RPM / 1351
For this example, Watts = 480 in-oz * 600 RPM / 1351 or 213 Watts.
213 Watts is servo territory. You have to use a servo motor to get that, about a NEMA-34 one.