Should all lines be the same size? I am doing 1.5 inch pipe.... My skimmer has 2 holes in the bottom of it.... One fits a 1.5 inch threaded pipe and the other fits something bigger... Should I run a bigger return pipe than the inlet pipes? Like 2 inches from the skimmer?

All lines do not need be to the same although you will have higher energy efficiency (more flow rate) with larger lines. If you are going to use different size lines, then use the larger ones on the suction side because you always want to keep suction head much less than return head to avoid air leakage and cavitation problems. I would recommend 2" as a minimum and 2.5" if want the system to be a little more efficient (~4%). Use separate suction lines to the equipment pad from each main drain pair, if you have one, and from each skimmer, if you have more than one.

Also, the size of the skimmer ports is not really that important since you can always use an adapter to step up to a larger pipe size. They also make skimmers with larger ports if you haven't purchased them yet.

I agree that bigger pipes are better but since it's there, using the 2" port if you up size the pipe is a sound practice.

In practice, bigger suction than return lines are encouraged. With suction, bigger pipes = slower moving water. This is especially important for submerged suction lines like drains but not as critical for surface suctions like skimmers.

Scott