The iPhone 7 and iPhone 8 are almost exactly the same size. A case from one would fit on the other. However, if we’re looking at the exact specs there is a difference.
The dimensions of the iPhone 7:
138.3 x 67.1 x 7.1 mm (5.44 x 2.64 x 0.28 in) (H x W x D) and 138g (4.87 oz)
while the iPhone 8 measures:
138.4 x 67.3 x 7.3 mm (5.45 x 2.65 x 0.29 in) (H x W x D) and 148g (5.22 oz).
So, the iPhone 8 is just a fraction larger and weighs a bit more; mostly due to the glass back rather than aluminum. Both the iPhone 7 and iPhone 8 have a 4.7 inch screen.