Calculations for uneven ground or inclines or soft wheels is complicated.
But for flat, level/even, hard ground it's very easy. With hard wheels (that don't deform very much) on flat, level/even, hard ground (where all wheels are always touching the ground) , the rolling friction (force on the edge of the wheels, not the torque) is probably less than 10% (for flat ground with no incline, this is 10% of the total weight). On an incline, it's X% of the normal force (the normal force is the the component of the weight that is 90 degrees to the incline) plus the component of the weight that is parallel to the incline which wants to pull the robot down the incline. Since it's flat ground and all whels always touch the ground, two motors means you can move twice as much. For reference, outdoors on rought terrain with soft wheels might have a rolling coefficient of friction of 0.3 (30%).
Larger wheels means you a longer radius which is like a longer lever that the motor must turn. This means less force on the edge of the wheels so to move the same weight you need a motor with more torque (or a gear ratio that gives you more torque). But larger wheels also travel more in one rotation and can go over bumps more easily than small wheels. You can get the same speed by using a small wheel and high speed, low torque motor or a large wheel with a low speed, high torque motor.
Just don't use the stall torque rating from the datasheet. The continuous operating torque is what you need (or rule of thumb is 1/7th of stall torque for best efficiency). Using what I said above (flat hard ground, no incline, hard wheels) you could probably move a 40kg robot in your case.