How do I change the pipe diameter and length ?
You have ‘scale’ operation on geometry in SimScale, but it is uniform. You will be better off using your CAD program, creating you own pipe (a solid) and importing to SimScale in right scale.
It is not clear. How do I use this to get a pipe diameter of 6 in and a length of 25 ft.?
The scaling my colleague @Retsam mentioned only gives you the option to have it for example 10x or you can decrease the scale by a predefined factor. If the scaling does not help to get both dimensions right I would advise to fix the dimensions directly in your CAD program.