The diagonal measurement of a square is (approximately) 1.414 times the length of the sides.
1.414 is an approximate measurement, but close enough for most quilting purposes.
Examples: The diagonal length of a (finished size) 9 inch square is 1.414 x 9 = 12.726 inches
1 x 1.414 = 1.414
2 x 1.414 = 2.828
3 x 1.414 = 4.242
4 x 1.414 = 5.456
and so on
For quick approximation - you can "think" 1.5 times the side length for the diagonal measurement.
How big your quilt top would be? That would depend on how many blocks you have. When you cut the setting triangles, you need to allow for seam alllowances.