Hey Charles,
If you are drafting the mainline in the first place and want it to only list as one size, you should adjust your Project Pipe Data to match that. For example, if you are drafting a 2" mainline for your asbuilt, remove all sizes other than 2" from the mainline class you are placing. Then, there is no need to adjust anything; you already placed a 2" mainline. Just add pipe labels where you want to.
If you do not see all sections of the mainline swap after completing the window portion of the Mimic Equipment command, that is because the mainline you have placed varies in its properties and data, either in size, class, or both.
I would recommend doing a simple test off to the side to understand this better. Place three legs of mainline. Use Edit Equipment to change their pipe sizes to three different sizes. Then, use Mimic Equipment from one size to another, and then window across all three. You will see the functionality work as described in my first post.
As always, if you think you are experiencing something different, open a Support Ticket and send in your drawings. We'd love to take a look.
Have a good weekend!