Choosing components for a triac's snubber
Please specify load characteristics and application. Very desirably provide a circuit diagram or at least a good word picture. 

The snubber's role is to dissipate transient energy which is looking for a home at switch-off time. We need to know how much energy there is, and how it is "stored" to know what sort of home it needs. As a guide you want an energy sink that will stop reactive voltage rising dangerously high, that will dissipate available reactive energy prior to the next switching cycle and which has minimal possible effect the rest of the time. Not magic, just compromise. 

Note that as your chosen optocoupler is "random phase switchable, you will have greater need of a snubber than if you were zero crossing switching - subject to the usual warnings re effects of inductive load currents not being zero at zero voltage. 


Datasheet BT139 TRIAC
Datasheet MOC3051M / MOC3052M optoisolated Triac driver 


To "start you on your way" ...


ST AN437 - 
RC snubber circuit design for TRIAC
Fairchild AN3008 - 
RC Snubber Networks for Thyristor Power Control and 
Transient Suppression
Related Fairchild AN3004 - 
Applications of Zero Voltage Crossing Optically 
Isolated Triac Driver
ONSemi AN1048/D - RC Snubber networks For Thyristor Power Control and Transient Suppression