Trolling reels are made with big, long running fish like Marlin in mind. They are designed to withstand a beating, and ask for more. They do come with a hefty price tag, but when you're chasing record sized big game fish, you may want one.
Like all fishing reels, Baitcasting reels can be categorized into size ranges, and they encompass a large range of fish, big to biggest.
As a generality, the following could be said regarding reel size to the line size:
Light - 16 to 30 lb test line; good for fishing King Mackerel, Dolphin, Sailfish.
Medium-Light - 30 to 50 lb test line; good for fishing Bull Dolphin, Wahoo, Amberjack, Sailfish, etc.
Medium - 30 to 60 lb test line; good for fishing Bull Dolphin, Wahoo, Amberjack, Sailfish, etc.
Medium-Heavy - 50 to 80 lb test line; good for all the big fish and Sharks.
Heavy - 130 to 200 lb test line; good for fishing all of the big species of gamefish, World Record Marlin, Big Sharks, Tuna, etc.