Rolex gift too tight

[zevy01]

So a while back I noticed my father hadn't worn his late 80s stainless submariner in years. When I asked him for it he laughed, naturally. I used to be significantly overweight, so I made a deal with him. I lose 100lbs and he gives it to me. And so I did . I just got the watch and it is too tight, my father is much smaller then me Im 6'4 hes maybe 5'11. Anyway Im very new to even the concept of watches I never used to wear them. Do I need to buy more links? I dont want to buy aftermarket only Rolex but Im willing to buy preowned. I also have heard if I buy from Rolex directly it will be very expensive and I cannot afford right now Im a college student. Also I noticed the links come in different sizes, which do I need? Lastly I noticed the watch has this extension in the clasp that comes out, when I pull it out the watch is too big but I dont like the look of the watch when that piece is out, what is it for? is it used to extend it?