In real life, an 1-3/4” handline can comfortably and safely flow from 100 to 180-GPM on the fireground, and any flows over 150-GPM are best run through low pressure combination or smooth bore nozzles to reduce nozzle reaction forces. Also keep in mind that at flows over 150-GPM, you will need at least three firefighters to properly advance the line and handle the reaction forces.
If you plan to use a 2-1/2” attack line, keep in mind the size and weight of the hose makes it very staffing-intensive, especially when used offensively. With practice, two members can deploy an uncharged line, secure the business end and flow 250-GPM for defensive operations with relative ease. The problem comes when you want to move it—there is just no getting away from the fact that you need 3 or 4 members to pick up the line and get it where it needs to go. Also take a close look at your business end hardware. A fog nozzle operating with a 100-PSI nozzle operating pressure and a brass playpipe might not be the best choice for easy and safe handling.
Keep in mind that a through, 360-degree size up must happen before you can properly select the right sized hose line. Pulling a line should not be determined by, “that’s what we always use,” or “it’s easier to handle than a 2-1/2”. Your people should recognize when more water is needed and be trained enough to be comfortable handling whatever line is ordered into service.