paperdiva, do you have the Cake Bible? If so, consider multiplying the base recipe on p.491 by 7 or 8 for your batter (either will work, it just depends on how high you would like the layer). Then for the leavening, multiply 1.25 teaspoons by the same factor, 7 or 8. I got this from the table on the following page.
If you are looking at the chart on p.490, please be aware that there is a correction- the baking times for the two sheet cakes should be reversed, i.e., the longer time is for the larger cake.
The thing about leavening and butter cakes is that the batters need to be stronger to span a longer distance between pan sides (kind of like a bridge needs to be stronger to span a longer distance between shores). An easy way to strengthen butter cake batter is to reduce the leavening.