Some basic questions about bodybuilding. Anyone care to help me out?!

Hey whats up. I used to work out about 4 years ago and I gained some nice size. I then stopped working out and gained some fat and look weaker. I just started getting back into this recently. Now the question I had was this....If I start getting used to working out again and I do not feel sore anymore is it okay to LOWER the weight so I can fit more reps in one set? I was thinking that it may make me feel sore again but then again I wont be able to gain size that way. (or will I?). Basically I am just tryign to get where I was and then a little bigger than last time WITHOUT taking any drugs like steroids or any supplements. If anyoe has some basic tips please let me know. Thank you so much.