Hi please could you tell me how to work out the size required for the stepper motors ? which kit to get thanks

The size of motors required for your machine build will depend on many characteristics of the machine. If the machine uses a gantry (rather than moving the table bed) and the weight of the gantry (specifically with inertia), The mechanics used with the axes, lead screw, roller chain, timing belt or rack and pinion. Generally you will need less torque if using lead screw due to the mechanical advantage, but friction is important to consider. If you are building a 4'x8' or larger machine, it would be best to use: https://buildyourcnc.com/item/electronicsAndMotors-3axis-heavy-gantry-elcombo Otherwise: this electronics: https://buildyourcnc.com/item/electronicsAndMotors-3axis-425-elcombo should be fine for most configurations. If you feel that you need extra torque on the z-axis (the z-axis will use a very heavy spindle, for instance), use this electronics: https://buildyourcnc.com/item/electronicsAndMotors-3axis-651-elcombo