Ok Joy, lets assume 82 inches wide divided by 6 block is approx 13.4 inches but we'll call it 14 inches finished for ease of calculation. This will give you a quilt 84 x 98 with 6 blocks by 7 blocks. Here are the approximations given to me by EQ7:
Please keep in mind these are only approximations but they should give you somewhere to start. You'll have to make adjustments based on exactly which log cabin pattern you are using and the size of your logs.