They have marks on the wall at some of the entrances and check the size against those sometimes. My current one which is 12x6ft has got in fine but I think it's technically too big. My first one which is 15x8ft was never allowed in. Only other grounds that have taken that stance are Rotherham, Shef U, Donetsk and Ljubljana (had to leave it outside on the street). It's rare a ground will say they aren't allowed tbh.
Best advice is take it anyway. Take your fire certificate or if it doesn't have one take it to the fire station and they'll give you one (a tenner I think). If you're not allowed in with it Wembley will either store it securely or you could try to find someone that has a hotel room, coach or car nearby that'll lock it away for you.
Something that should be noted is in the lower tier the way how the barrier at the front of the seating is designed means it can't hang vertically very easily. In the upper it also can't hang below the line of the advertising boards. At the Scotland game a steward helped me put ours out. However during the first half another steward untied it (it hadnt dropped but it was hanging too low apparently) and left it on the floor which meant everyone walked on it at half time.
