I'm honestly not sure if you're joking but in case you're not, the "minimum viable size" is hardly the largest issue with what you're proposing and it sounds like you're not getting what the key issues are.

You're talking about taking a technology that's so finicky we've barely gotten it to work after almost 100 years and rocketing it into space? We're no where near good enough at this to get a test that would work after the extreme violence of an escape velocity launch.

Further fusion reactors aren't like fission. "exploding" really isn't a problem . Keeping the reaction going in an efficient manner is.

IF exploding was a problem, space is probably the worst place for it? Putting it way underground would be vastly easier and a hell of a lot safer because you won't have material possible falling back to earth/hitting satellites in orbit.