Size should play no role in soccer if the coach actually knows what they are doing and the players have decent skills. Where I do see a difference is on the lower level teams that use the big players to push around the smaller less skilled players. Once those big players encounter skilled players their advantage is neutralized. I think that some coaches do a disservice to those younger big players by just putting them in a defensive position and just telling them to clear the ball. I saw it last weekend while refereeing 10U girls and boys at state cup. Those big boys and girls were all put in that center defense position with one mandate, clear the ball out of the back. The smaller players had trouble getting around them initially, but soon learned to use speed and pass around the big player.
My 5'2" daughter played in college against women a foot taller than her. She had no problem with the size difference because she had the skills needed, had speed, and was strong. She would be in the weight room 4-5 days a week to make sure that when those taller girls bumped into her they were the ones bouncing off. Growing up she was normally one of the smallest players on the team, but she always worked hard to make up for her size by having superior skills and speed compared to the bigger girls.