Have you tried to follow Sheldon Brown's website description of bicycle gears? Sheldon Brown's Bicycle Glossary G

The largest rear cogs are the easiest to pedal, as in up steep/long hills. It's the smallest cogs that are for down hill and high speed use.

But to answer your ring teeth question. 48/34 is about 1.3. So a 25 large cog would need a 32.5 tooth ring to mimic the 48/34. Andy