Hi Sashasmommy, I love to make large cookies as well and here is what I've learned.
1. I cut the dough 1/4" thick as I do my smaller cookies.
2. After I roll out the dough, I roll it onto my rolling pin and unroll it directly onto my Silpat Mat and THEN cut it. This keeps the shape of the cookie intact rather than cutting it on a board and transferring it.
3. Like Econlady, I do transfer the cookie to a cake board but a couple of things to note as you do this. I let the cookie cool completely. After coming to a room temp coolness, I put the cookie, still on the pan, into the freezer for a few minutes. I then take it out of the freezer. When removing the cookie from my Silpat Mat, I try to peel the mat off of the cookie, rather than lift the cookie off of the mat.
4. Prior to placing the cookie onto the board, I lay the cookie cutter, or paper/cardboard design you used to cut out the dough, on the board and mark about 6 places where I place a dab of icing on the board so the cookie does not move while I decorate it.
5. Once on the board, I never touch the cookie with my hands again. I take the board with the cookie adhered to it and place it on my "lazy susan" where it stays as I decorate it.
6. With regard to the consistency of the icing; I use a piping consistency to outline and a flood icing to flood. Especially when the cookie is this large, you will need to have a solid barrier to keep the icing from flowing over the edge. Why take the chance in that happening? The idea is to not touch the cookie once it is placed on the board.
Hope this helps. I love this particular cookie idea! Good luck and please share it with us when you finish it! ~Diane, Cookie Celebration LLC