UCM....the maximum size of the journal must be no bigger than 8x8. The most popular size is 5x7 or 7x7 square. So noone is doing a 12x12.
I certainly hope that the journals will not get tatty. They should be treated with respect because they are after all someone elses journal.
I have done several and never had a tatty one turn up because everyone appreciates the time and work that is involved in making the journals in the first instance.
It might be worth bearing in mind that if you have any special embellishments that you want to use, that they are adhered well and that they are not to bulky. If they are then maybe it would be better not to fix them to the covers of your CJ until it has been completed by your team mates.
HTH.