A column-inch is a measure of space one column wide and one inch tall.
Columns in the ICO are 9.5 picas wide, with one pica between columns.

Ad sizes in inches and decimal equivalents are as follows:
1 column (9.5 picas) 1 9.16 inches (1.58")
2 columns (20 picas) 3 5/16 inches (3.33")
3 columns (30.5 picas) 5 1/16 inches (5.08")
4 columns (41 picas) 6 13/16 inches (6.83")
5 columns (51.5 picas) 8 9/16 inches (8.58")
6 columns (62 picas) 10 5/16 inches (10.33)