I had sex for the first time last weekend and his ***** wouldnt go all the way in me? Why not?

Your body size had nothing to do with it. Since it was your first time I'm assuming you were nervous and maybe a little scared, the usual. Those feelings cause your body to tense up causing your vag to close instead of expanding to let him in. You also won't be producing as much fluids to lubirate causing it to be hard to move. Don't force yourself to do it if you don't feel comfortable because you will be really tight and dry and it will hurt

As with most things, practice makes perfect so it may take a few sessions for your body to adapt to his size and techniques.