I have been to northern Maine on 3 trips. I have taken 2 bear. First was about 280lbs and the last was 430lbs with a 18inch skull that was entered in the Maine book for Muzzleloader. Maine does have plenty of smaller bear, but it has some nice ones too. I think that a lot of it depends on the Guide service you use and the land they hunt, plus your patients to old out for a good bear.
The season in Maine starts the week (Monday) before Labor Day. You could connect in 3 days, but that is never guaranteed. I would definitely stay the week. I connected on that last one on the last evening of the hunt. Keep in mind that a lot depends on the mast crops during the year as to how many will come into the baits too. Heavy mast crops will keep the bear from coming into the sweet stuff, so you need to be patient. How good the mast crops are is something you will not know when you make your reservation, But thats Hunting too.
I personally recommend BearCreek Lodge. John Schmidt is a great guy, very knowledgeable on Black bear and has a lot of Timber company property leased as well as local farms. I am planning to go back next season myself. I would not think of any place else to go.
http://www.bearcreeklodge.net/