Hi, if wanting to build big muscles how should the reps and weight go?

Ive heard some say you should start with a lower weight for 10 reps then add a bit for, say 8 reps then finally add more weight for as many reps as possible.

On the other hand ive heard people say that you should select the hardest weight and stick with it for 3 sets lifting until failure in each set regardless of reps? is there a rule??

7 to 10 reps works best for building size - but it has to be real quality, working to failure on each set.  Simply doing 7 easy reps won't do it for you.