"I beam" sizes are standardized by the American Institute for Steel Construction (AISC) and range from 8 inches to 44 inches in depth. Each beam depth comes in a variety of widths and thicknesses.
I beam is the generic term that most people use for any steel beam with a cross section that looks like the letter I. However, the construction industry refers to these as "W shapes." This is because the proper name for these beams from the AISC is "wide flange shapes." There are also "M Shapes," "S Shapes" and "HP Shapes," which have a cross section that looks like the letter I, however, W Shape members are the most common in the building industry.
When choosing a beam size for a new structure, or evaluating one in an existing structure, always consult with a licensed structural engineer.