We transitioned early in our club fall of 2016, recreation and travel/select we implemented the Birth year and the field sizes and build out lines, the key thing is to teach your receiving defender to be deep near the goal line when receiving and have your keeper be be able to receive the ball back and switch the point of attack. When using the build out line as offside all this is doing is creating space on the field rather than having all 13 players in a small area. I think USSF implemented the offside rule from the build out line back was down to the original field sizes which were 30x47 at the u9-u10 age group. the old office rule where the midfield denotes the offside would mean that if a team pushes up and presses you would have 13 players inside a 23.5x30 space 705 yards2, With the build out line as denoting offside 33x30 then the 13 players had 990 yards2 But when USSF had push back on the field sizes they changes them to be in a range at the 13 players between build out line and goal line smallest 41x35 which gives 1435 yards2, largest 51x45 which is 2295 yards2, If we compare this the the 9v9 games and the 11v11 game 9v9 17 players in one half of the field using 1/2 field as the offside denotation. smallest 35x45 1575 yards2 largest 40x55 2200 yards2 11v11 21 players in the opposition half 1/2 field denoting offside smallest 50x50 2500 yards2 largest 56x100 6500 yards2