Depends on the species, some will grow considerably larger than 5 inches. If it is a warty frogfish you should be right though, they get to around 4 inches.
In general though, I would suggest investing some time into researching saltwater basics before trying your hand at an actual tank. Frogfish are notorious for being hard to acclimatise when first added so you will need as much knowledge as you can muster. They can be weaned onto frozen foods but it is a bit of a hit and miss affair with frogfish, some switch over ok but many do not. Corals should be safe with frogfish but anything else up to one and a half times it's size is fair game to a frogfish, often swallowing as much as it can while the rest hangs out the fish's mouth waiting it's turn. Because of their feeding habits they require strong filtration.
I won't add anything else as aquascaper's recommendations fill in the rest for you.