Weight is deteremind almost exclusively by your diet; what, when and how much you eat. As far as training for strength gains you will want to stay in the 1-5 repetition range for each set, limiting the total number of reps for each exercise at around 25.
Example - Deadlift 5 X 5
-25 total reps, each set should be as heavy as you can handle
Strength gains are made primarily through the nervous system and its ability to recruit muscle. To train this, extremely heavy weights (relative to your maxes) must be used.
Size gains are made through the muscular system in a response to tissue damage caused by incredibly high rep totals. This is why we see bodybuilders doing upward of 100 reps per body part, their goal is to shred their muscles to elicit that growth response.