You need to size your motors correctly depending on how much load they are expected to overcome. You first consideration should be how much maximum force would your robot need to exert at any given time....
For a robot just starting to drag a load (assuming the surface is flat) behind it, the robot would have to at least apply a force just greater than...
Maximum Static Frictional Force due to Robot's own weight on its wheels + Maximum Static Frictional Force due to object being dragged.
Maximum Frictional Force = Normal Force (weight) * coeff of Friction for the two surfaces
So for example, the coeff of friction between a brick (load) and ice (surface) would be lesser than between a brick (load) and asphalt (surface).
You need to do these calculations first.
If the speed limitations of your robot are inconsequential, then you only need to choose your wheel size and then determine how much torque your motor needs to be able to generate.
You do this like so.....Motor Torque needed = Force to generate / (radius of wheel * number of motors you want to install). So for instance, the torque needed per motor in a 4WD robot would be half that needed for an equivalent robot with only 2 powered wheels.
Once you have the motor torque you need, you can choose a motor and then choose a suitable power source for this motor.
Now repeat ALL the steps above this time accounting for the weight of the motors and the battery.