Sena,
If you're making your own molds the dimensions are totally up to you.
First, calculate the cubic area you'll need. The density of copper is 8.96. We'll round that to 9 to make things easy. So each cubic centimeter (1 cm x 1 cm x 1 cm) weighs 9 grams. For a 10 kilogram ingot, you'll need 10,000 / 9 = 1,111 cubic centimeters. So, just as a rough starting point, a 10 cm x 10 cm x 10 cm mold would be 1,000 cubic centimeters, a little smaller than needed for a 10 kilo ingot.
From there you can just play with some numbers. Cut the thickness in half to 5 cm, and make the length 20 cm. 5 x 10 x 20 = 1,000 cc. Make it a little longer, say 22 cm and you'd have 5 x 10 x 20 = 1,200 cc. That would be just about right, allowing a bit of extra so the pour doesn't overflow the mold. Adjust the numbers to suit your desires.
Don't forget to provide a slight angle, or draft, on the sides so the ingot dumps right out and doesn't get wedged into the mold.
Half that for a 5 kilo mold.
Hope that helps.
Dave