Master's of Science
The cork master is the master of the cork because he knows which wine goes with fish or pork...
masters is multiple masters and master's is possession:"that's the master's magic wand."
"John Doe, who has a master's degree in psychology." is the proper usage of the apostrophe. Also: you don't capitalize "master's"
It depends... A master bath can be as big as the person who built the house wanted to but the for the master bathroom to be a master bathroom you have to have a sink, toilet, and a shower/bathtub