Either as big as you want or no more than Chernobyl scaled for the energy output
Fusion reactors don't exist.<citation needed>
Several of the answers are using the present-tense to explain what they think a fusion reactor will do. That's amazing, since you don't explain anything about the reactor. You don't explain its nature, its size... nothing. Therefore, you really only have two options:
1: How many angels can dance on the head of a pin? As many as wanting.
When it comes to undefined reactors rigged to explode, your best bet is to simply define the damage that you want and move forward.
2: Use Chernobyl as a point of reference.
Yes, Chernobyl was a fission nuclear reactor. It's as close as we can get without an actual fusion reactor. But, let's start with a truth:
Myth # 2: A nuclear reactor can explode like a nuclear bomb.
Truth: It is impossible for a reactor to explode like a nuclear weapon; these weapons contain very special materials in very particular configurations, neither of which are present in a nuclear reactor. (Argonne National Laboratory)
But this doesn't mean a reactor can't explode...
Nuclear reactors can't explode like nuclear bombs. But they can explode, because an out-of-control fission reaction generates a boat-load of heat. Build up enough heat, and everything from water to metal vaporizes, creating pressure. Get enough pressure, and you get a boom.
Yeah... but how much of a boom?
And we're back to having no idea the type or size of your fusion reactor. Don't know how it's designed, what safety features are in play, we don't even know what fuel your reactor will be using, but we can take a guess:
The current best bet for fusion reactors is deuterium-tritium fuel (U.S. Dept. of Energy)
Deuterium is chemically identical to hydrogen (Source). So it's no more dangerous than hydrogen.
Tritium is also chemically identical to hydrogen (Source), although it is a radioactive isotope. The radioactivity could be a believable problem, but the referenced source suggests it would take a lot of it.
Having said that, hydrogen can explode. The Fukushima nuclear power plant experienced a hydrogen explosion. How many people died? None. Five workers were injured. Yes, hydrogen "explodes," but it doesn't explode like TNT or nuclear bombs. Think about the Hindenburg. When it exploded there was a massive fireball — but there wasn't a glass-breaking concussion.
Now, the Hindenburg wasn't contained like a concrete-and-metal encased reactor — but it did have access to abundant oxidant. Whether or not a fusion reactor had access to significant oxidant would be very circumstantial.
I could be wrong, but I don't see a fusion reactor doing anything more than what Chernobyl did. (And, realistically, it might even be safer than Chernobyl due to the lack of all that radioactive fuel — uranium.)
1 We're not talking about a star here. A large-mass star will burn hydrogen and helium. When it's gone, they'll burn carbon. If they're large enough, they'll burn neon after the carbon. But once a star gets to iron the process of fusing iron absorbs energy. But we don't have that mass. So when a fusion reactor and its fuel mass loses containment the fusing part of the process would die very quickly, leaving the usual nasty mess.