The most frightening . . .
For me, the most frightening size for the tarantula was when we saw it creeping around inside the glass case near the beginning of the film. It was only (!) about 4' in diameter, and much less destructive than what it eventually became, but scary as hell in this smaller size.
When a creature is truly gargantuan (as near the end of the film), it's scary mainly--or solely--because of its size and sheer destructive power. At that scale, it doesn't really matter that it's a spider. It could be a 100' tall chipmunk.
So, I think something is lost, from a horror perspective, when what is naturally horrid to many people (e.g., a spider) is excessively scaled up.
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