Howdy,
Mercer is owned by Dalton Rock in Dallas.
You can call them. But, I believe that they told me about a year ago...that motor vehicles of any kind are PROHIBITED. I also believe that you can walk in or ride a bicycle, but it's a windy and "washboardy". And I also think it's 6 miles to the lake! (BUT please get permission to enter the property from the Dalton's...DON'T merely take my word for it).
I'm planning to fish Rickreall Creek on the opener, on May 22nd (below and above the gate). It's open then for 2 trout per day on artificial flies/lures. And there are some NICE cutt's in there! Keep in mind, that the creek is NOT open right now. But, it looks like Mercer Res. is open year round.
According to "Fishing in Oregon" (7th ed.), it is 60 acres in size. And the book says it has native cutt's up to 12". But, I'd guess it's lightly fished and had much larger fish in it.
Good luck,
TD