There are other ladies on the forum who have been using Epsom salts for longer, so hopefully they will be able to provide better advice. I started off by using a tiny amount, in case it irritated my skin, and have since increased it to a small teacupful in a fairly full bath. You are advised to rest afterwards for at least 2 hours unless you have arthritis, in which case you are advised gentle movement.
I don't drink any more on days when I have the Epsom salt bath than on other days, but I do make sure I am well hydrated all the time. Before I tried the baths, I looked at a few articles online and I did find a couple of sites that were talking about the baths helping you lose weight as they suck all the toxins out of you!!! These sites advised adding loads of salts to the baths...maybe the site you looked at was similar? I can imagine if you used 5 or 6 cupfuls, it might dehydrate you, but I have been fine with just one cupful.