API 5L X42 is a popular grade of line pipe steel widely used in the oil and gas industry. The most commonly ordered API 5L X42 pipe sizes depend on the specific application and project requirements. However, some commonly requested sizes include:

1. Seamless Pipes: API 5L X42 seamless pipes are often ordered in sizes ranging from ½" to 36" (DN15 to DN900). These sizes cover a wide range of pipeline applications.

2. Welded Pipes: API 5L X42 welded pipes are frequently ordered in sizes ranging from 2" to 48" (DN50 to DN1200) for various pipeline construction projects.

It's essential to note that API 5L X42 pipe sizes may vary depending on regional standards and specific project specifications. Factors such as pressure ratings, wall thickness, and intended use will also influence the selection of pipe sizes. It is recommended to consult with suppliers or adhere to industry standards and project requirements to determine the appropriate API 5L X42 pipe size for a given application.