1 inch is standard size for drains and 3/4 inch is standard for returns.
You can go bigger if you wanted to, but usually not much point. Bigger drain will be louder as well. The main reason to run a bigger drain if you needed more the 900gph drain per overflow whole. Meaning a typical 1 inch drain can handle draining around 900gph. So if you have two drains you can handle draining up to ~1800gph. If you don't need more then that I would stick with 1 inch. If you are pushing more then 1800gph through your sump then that probably way overkill as that doesn't leave a lot of "reaction" time for things like skimmers, reactors, UVs, etc.
Also keep in mind that even if your pipes are 1 1/2 inch but it downsizes into a 1 inch bulk head then the drain will still only handle 900gph because of the bulkhead size. Either way just make sure your drains are always bigger then your returns.
Hope this helps.
Jakub
