The stock tyre size for Beat Petrol LT is 155/70 R14. If you want to upsize it then go for 165/70 R14. MRF ZLX would be a good tyre to begin with. In case you want something better then you could look at Bridgestone B250 or Bridgestone Turanza AR20 ( our recommendation being Turanza if it fits your budget).
You could also look at buying Yokohama Earth-1 tyre. It is better than Bridgestone B250, although not as good as Turanza.