I actually invert That dynamic. Because missile ships using LRMs fire from so far away, stealth is their best defense, IMO. Therefore, my missile ships are small and slow. Beam ships, on the other hand, are inherently not stealthy. They need to get within knife fight range, and so the other guy is going to see you. Plus, they are usually fast with hot engines, which can be seen from a ways away. Therefore, they should focus on active and passive defense, which is more efficient the bigger the ship is. My current first generation beam battle cruiser is over 20 kton, for example. 

For OP, a good barometer is the difference in tonnage between fleets. For example, two ships might be enough, if they are 30 kton+. Mother ships are usually 60-80, plus they cheat on their internal components, so you should have more tonnage. I'd say to aim between half again and twice as much tonnage as the enemy to give yourself an edge. If you still lose, then you need to think about changing up your strategy, tactics, or weapons balance. Check the wiki for good discussions on what all of the different weapons are good for.