Stop messaging app converting texts to MMS

The Samsung message app converts long messages to MMS which ends ends up costing me. Can this be switched off? The Hangouts messaging app doesn't do this but I prefer the look of the Samsung app (personal preference).

Does anyone know how to stop the Samsung app from converting long messages to MMS?

Question about converting back to sense.

Would it cause problems if i flash the rom before i do a factory reset through stock recovery??