2001 Impala oil leak

I would immediately check the drain plug and filter to make sure they are secure. If the oil change place left one of them loose it might be ready to fall out completely and blow your engine. A leak of that magnitude should be easy to spot and is likely to get worse fast.

Thanks. So I don't dare to use the car before taking it to dealer.

I can just look at the engine from top-front with hood open. can only see the filter and the engine to its right side (covered by oil film). Seems the leak starts from the filter or a close spot. Apparently much thicker oil film on engine around the filter. I'd feel relaxed if it's not the gaskets blown again.....

Simon, in your original post you said: "Now the oil stick level is a bit lower, the coolant level is just right." This would suggest that the leak is not major, considering you just finished up a 1500 mile trip. Oil leaks can be deceptive and while you said there was "a big puddle of oil on the garage floor right beneath the engine", I suspect that the oil probably dripped down from the engine block and filter and spread out a bit, exagerrating the size of the leak. It really sounds as if the filter was not properly tightened. If the dealer who did the oil change is in your area, I'd simply drive the car back to them and have them tighten the filter and top up your oil (free of charge, of course!), so they know that they screwed up. If the dealer isn't nearby, tighten it up yourself if you have the equipment and top up your oil, failing which, just take it to the nearest garage and have the filter tightened and the oil topped up.