Ah questioning supplements at 15, one of the greatest fallacies of Rugby.
The reason you even firstly question whether to use protein or not comes from your self-stated lack of ability. Playing against big people shouldn't be an issue if you have good enough technique. Yes, it will be scary the first few times you encounter that man-child that we have all encountered, but get your technique sorted and you will turn over anyone worse; regardless of their size.
Also, as Ranger and others have pointed out, protein shakes are not amazing growth hormones; they just aid muscle growth. You still need to do the exact same amount of work to get strength. At your age if you want to improve you only really need to be worrying about improving your strength - not mass. That will come naturally.
I would advise you
not to do protein, especially at your age. Too many kids think this is the key to greatness - hell, even I took protein for a few months when I was about 17 but I realised after that I was using it for completely the wrong reasons. I wanted to get bigger, mistaking it for a synonym of 'better'. Really, you can only ask yourself what you really want. If you just want to get bigger and be exactly the same player; being bowled over by that same Goliath again and again, then by all means take protein. If you actually want to improve and tackle the real issue you need to train hard on your technique. Once you become a better player and a bit older then you can start exploring weight gain.
This tells you everything you need to know:
yea keith the problem wasnt that our team arent on suoplements its that were ****