I too (occasionally) cycle to the game. The club should provide a safe storage facility for bags exceeding the size threshold. These can then be retrieved after the match. I too was told my pannier was too big (no flask or butties in it either!) but it does have spare inner tube, pump, tyre levers, keys, overshoes, waterproof trousers, front and rear lights and helmet and gloves in there when going through the turnstyles. To try and fit all of this stuff into an A4 bag is tough. It's quite hard to find any bike panniers which are less than A4 in size as their smallness would render them useless tbh.