The more you take, the stronger this effect will be up until a point. After that, any extra grams will likely instead just cause drowsiness. I find it's best to take a medium to large size dose. This should last around 3 hours. There's also the option to take a gram every hour after to prolong the effects.
If you don't know how much a small or medium dose is, a small is usually 2-3 grams and a medium is around 4-5 grams. The size of dose varies a lot though, since kratom sensitivity and tolerance is different with everyone. It's best to experiment at first with kratom to find the appropriate dose sizes.