Do you want to use this to make stop-motion video using a still camera, or do you need to be able to dolly a video camera while it is running? The former is a pretty easy task because you can let the whole rig settle for a second between moving the dolly and tripping the shutter; the latter is much more difficult in terms of vibration and you need very smooth motion.
Anyway, to expand on MarkT's answer, torque defines how much force is available from the motor.
Torque = Tangential_Force * Radius
Say your whole moving platform weighs 5kg: to support it vertically requires 59.8 = 49N of force. Or to support it on a 60-degree slope requires 49cos(60) = 25N of force. Now say you need to accelerate the trolley at ~0.25G = 2.5m/s^2, take F=ma and that gives you a required acceleration (and deceleration!) force of ~32N. So if you want to operate vertically, your motor system needs to be capable of supplying 49+32 = 81N of force.
Now, you choose a combination of pulley size (gearing factor) and motor. Say you use a 15mm pulley, directly attached to the motor, that has a radius of 0.0075m. Use the above equation:
T = F * R = 81 * 0.0075 = 0.6 N-m.
Therefore your motor needs at least 0.6 N-m of torque at all times, which is a bit less than 100 oz-in. Here's where it gets non-obvious: the specification on a stepper motor is holding torque, i.e. torque while stationary. The actual torque available while moving is less, and how much less will depend on how fast the motor is going. Read this introduction to get you started. And remember that torque is mostly proportional to current, so if you're running the motor at half of its max-rated current, you're getting only half the torque.
So being conservative, you could run your system with a stepper motor of about 2N-m holding torque with 15mm pulley and it would probably be OK. Or you could pick a smaller pulley size: going from 15mm to 10mm means you have 1/3 less radius, therefore 1/3 less torque is required to achieve the same force, but you will get 1/3 less speed too.
By "probably" above, I mean you still need to pay (a lot of) attention to your motion planning. You need to plan your motion so that the mechanical load (e.g. due to gravity and attempted acceleration) is not greater than what the motor can provide, otherwise it will skip steps and you won't get the position you expected or it might just halt & catch fire.
Have a google for "grbl", which is some firmware you can load into an Arduino that does stepper control in 3 axes, with text input over the USB port. You command it a position, and it goes there, obeying specified limits (that you choose beforehand) on speed and acceleration. Using grbl with an external PC (or even another microcontroller would be fine) to handle the motion planning (when to move, how far and how fast) would be IMHO an excellent way to implement a camera dolly. Even if you don't use it, I recommend you go and read up on things like grbl because it'll tell you a lot of the things about CNC motion that you will need to know to implement your own system.
