Harvey means to set the larger dimension of the two, (i.e. 18 x 24), to 7 inches, which means, in the example, 24 inches would be the larger of the two dimensions and the one you would set for 7 inches.
When you make that change, the other dimension will automatically change itself proportionately.
David "The Stunt Engraver" Lavaneri
DGL Engraving
Port Hueneme, CA
