i hear this quite a bit from a friend of mine who just started lifting. i tell him not to worry...hes not going to go from 6' 150 to 6' 225 overnight.
dont have limiting thoughts when it comes to motives. if you want to gain size, tell yourself that you want to gain size. the gains will come slowly. you arent going to wake up one day 25 pounds heavier. when you are satisfied, you can stop trying to gain mass (although, if you are like me you wont be. i started at 150 and wanted to be 165. once i was 165 i wanted to be 175, when i was 175, i wanted to be 185, now at 190, im shooting for 200 by next year).