Well we have a guide on how to choose the best motor for your drone, but the general rule is to have motors that can produce 1.5 times more thrust that the total weight of your quadcopter. So if you have a tricopter that is 1Kg, you want the combined thrust of all the motors to be 2Kg. That will give you quite a responsive tricopter which will be fun to fly.
If you have any questions please let me know and I will be happy to help, if you let me know how much thust you need I can make a few suggestions in terms of motors and props.