Since the beginning of ages people have debated what is the
optimum amount of repetitions to build the most muscle
size.

The answer is not that simple since there is a slight
overlapping training effect between different repetition
schemes.

The question should rather be what you want to achieve.

To achieve maximum results don't mix up different
repetition schemes, although it can be done. You don't want
to mix up the specific stimulus you want to deliver to the
muscle fibres.

In short these are the three repetition schemes that you
might want to use and what effect they are going to have on
you're muscles.

1-4 reps (building strength)
5-8 reps (building muscle size)
9-25 and more reps (building muscle endurance)

Use only compound exercises with as much weight as
possible, which will allow you to work between 5-8 reps.
The moment you can do 8 or more reps, increase the weight
slightly.

Compound exercises are done with barbells and dumbells.
Forget about any isolation exercises. Isolation exercises
simply do not tax the muscle fibres enough to cause the
growing effect.

If you do have the choice between barbells and dumbells
take the barbells. Write this into your mind. The heavier
weight you can use in your exercises the more effective
you're going to build muscle.

Never use loose form. Although you are going to use heavy
weights, use strict form in your exercises to maximize
results and prevent injuries.

These are the best compound exercises for building muscle
size. Deadlifts, squats, leg presses, bench presses,
bentover rowing, normal or weighted pullups, military
presses, barbell curls, close-grip bench presses, standing
and seated calf raises.

There are of course other factors that also play a major
role in developing muscle size, but to many times I find
people fooling around with light weights and doing a
rediculious amount of repititions.

Start using the correct repetition range today and in three
months time your body might look completely different.