The size of steel joist you'll use is mainly determined by the load or weight being supported. Consulting the Standard Specification Load Tables for Steel Joists from the Steel Joist Institute should give you an accurate guide to the type, size and depth of steel joist suitable for your project.
The most commonly used Open Web Steel Joist has depth varieties of 8'' to 30'' and can span up to 60''. It has a 2.5'' standard seat depth and a uniform load capacity of 550 plf. The Long Span joist is useful for floors or roof decks and has a depth of 18'' to 48'', spanning up to 96'' with a uniform load capacity of 100 plf.
Deep Long Span joists are primarily used for roofs. They have a uniform load limit of 700 plf, with depths of 52'' to 72'' and a span of up to 144''. Both LH and DLH joists are available in a seat depth of 5'' or 7.5''. Loads may, however, vary at different points of the floor or roof deck due to design. You also have to consider factors such as joist span, joist spacing, direction and deflection to choose the correct type and size of steel joist.