"The Sick Rose" uses a strange meter called anapestic dimeter, meaning that, theoretically, each line should have two ("di") anapests. An anapest is a three syllable foot that has two unstressed syllables followed by a stressed syllable, as in line 7:
And his dark sec-ret love.
As it turns out, though, this is the only pure anapestic dimeter line in the whole poem. Many of the lines have what is called a substitution, where instead of one of the anapests we have something else, like an iamb (an unstressed syllable followed by a stressed syllable) or a spondee (two stressed syllables). Take the first line for example:
O rose thou art sick.
You'll notice that the first foot has two stressed syllables (spondee), followed by our expected anapest. Line 2 does the same thing, only the first foot is occupied by an iamb rather than a spondee ("The in-" counts as one syllable and is pronounced together like "Thin-"): "The in-vis-i-ble worm." Line 3 follows the same pattern as line 2 ("That flies in the night"). Line 4 also makes use of a substitution, but this time it's in the second half of the line: "In the howl-ing storm." The line starts with an anapest (two unstressed syllables followed by a stressed syllable) and ends with an iamb. Lines 5 and 8 do the same thing; they begin with an anapest and end with an iamb. Line 6 is the odd-ball here; it's just iambic dimeter ("Of crim-son joy).
Now, you might be saying to yourself, why should we even call this poem something fancy like anapestic dimeter when Blake makes all kinds of substitutions (remember only line 7 is purely anapestic dimeter)? It's a good question to ask, and the answer is that substitutions are perfectly acceptable in metrics; it makes sense to call it anapestic dimeter not only because that's a really cool name but also because it's the only way we can explain the fact that most of the lines have five syllables. Otherwise we would just describe the poem as "five syllable lines with no metrical pattern." That's boring, and doesn't do justice to the sophisticated metrical work going on here.