I want to cut a sheet cake 7/12 how many inches should each piece be?

It depends on the size of your sheet cake.  They vary.

Measure the cake, and divide appropriately.  If you want to cut it into 7x12 pieces, divide the short side by 7 and the long side by 12.

Some do 1x2 inch, some do 2x2 inch servings.

Here is a serving guide that should help with figuring out how many servings per cake: http://www.wilton.com/cakes/making-cakes/baking-serving-guide.cfm

And here is a cutting guide: http://www.wilton.com/cakes/cake-cutting-guides/party-cake-cutting-guide.cfm

Ah, I was thinking they wanted 7 pieces by 12 pieces, rather than that it was a 7x12 inch cake.  In any case, it's going to depend on part on how many people you need to serve, and/or how much cake they want.