The only disadvantage of to big is heaver than it needs to be.  Speed controller needs to be rated with a high enough amp ratting to handle the motor battery prop combination you plan to use.  Look at the specks for the motor you plan to use.  For a given number of cells their should be a max continuous current rating.  Speed controllers are also ratted in max number of cells.  This needs to be the same or higher than the number of cells you plan to use.   Other considerations are weather you need A constant rpm mode and if you want data logging.

RSM sells matched sets of components for various sizes of aircraft.  Not a bad way to go if you lack experience to pick your setup.