I've had great success pouring large wet plate collodion plates in 11x14- 12x15- and 12x20. John Coffer was the one who got me started in the process. What John does, and I find works very well, is thin your collodion with a 50/50 mix of alcohol and ether at about 10% of your volume of collodion. What happens with the larger plates is the ether is evaporating quickly and your collodion is starting to set up before you get your plate coated. Once you bring your plate up vertical onto your pour-off bottle rock it vigorously before you bring it back flat. Once your bring it back flat turn it and give the edge of the plate a little tap against the table the opposite way from the pour off corner. This helps eliminate those flow lines you see in some plates.