Toroids of the Tesla Coil have a number of functions.
1. It acts as a capacitance that resonates with the Tesla Coil Secondary. The larger the toroid, the higher value of capacitance. When the coil is operating, a cloud of ionized air builds up around the toroid and increases capacitance further. 
2. It sets up a smooth electrical field around the top of the Tesla Coil Secondary winding. Higher voltage levels can be reached before a spark discharge
3. It creates an electrical field around the top of the Secondary and keeps the air around the top of this Secondary from breaking down.
The shape that the top load usually takes is a sphere or a Toroid. The sphere is the theoretical ideal for holding the highest voltages without the surrounding air breaking down, but the Toroid gives an electrical field that better protects the top windings of the Secondary.  My toroid for the 900-watt coil measures 3 inch by 14 inch with a capacitance of 15 pf.  Tab on picture to enlarge.