I get a bit confused when to use commas before certain words. For example, either.

Is this correct?

Not the biggest boy in the world, either, at 5ft 2ins, he began working out...
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There are several rules governing the use of commas, but there is also a lot freedom. Webster's Third New International Dictionary begins their article on commas with:
Of all the marks of punctuation the comma offers the most difficulty in use and the widest range for individual choice. Though often marking rhetorical or elocutionary pauses, the comma is used primarily to separate or to set off in a group. It sometimes distinguishes nonrestrictive modifiers from restrictive modifiers.
More to the example at hand, they say:
Commas set off transitional words and expressions (as on the contrary, on the other hand, consequently, furthermore, moreover, nevertheless, therefore) whenever they are or would be spoken with the adjacent rising or sustained pauses that indicate subordinate matter <The question, however, remains unsettled.> <Nevertheless, we shall go.> <On the contrary, under the rules a vote is in order.>
I would say use a comma before either if you feel there would be a pause there if the sentence were spoken.

Either in this sentence is used as an adverb and means likewise or also. It is used as an intensifier following the initial negative phrase. There would have to have been a previous statement to justify either. Also, I think the word being is ellipted.
He was left weak by childhood illnesses. [Being] not the biggest boy in the world[,] either, at five feet two inches, he began working out.
To me, the use of the first comma is debatable. In this construction I would tend to omit it because either seems to belong with the rest of the negative phrase. Again from Webster's:
<You'll not go far in life and you won't be happy either. --W. J. Reilly>.
Thanks so much for the detailed reply. You have addressed my problem with 'either' (in my bad example). Essentially, I am never sure when to use a comma with 'either' near the end of a sentence or not.

Is there a way of working out when to and when not?
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Maybe you should say your sentence aloud first. If there is a pause befor "either", then you write a comma.
Since either can be a pronoun, conjunction, adjective, or adverb, and since there are so many ways it might appear in a sentence, please show me some the sentences about which you have questions.

I get hung up when to use commas with either at the end of a sentence...

No, I can't either.

No, I don't either.

I don't go mountain climbing and I don't go mountain walking, either.

I don't like peaches or nectarines, either.

You'll not go far in life and you won't be happy either.

Not the biggest heavyweight in the world, either.

Note the commas after some 'eithers' and not with others
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I would omit the comma, but I do sense a slight pause before a final, adverbial either. In that position, either is used for emphasis, and a pause before it adds to the effect. Perhaps the feeling that a comma belongs comes from the similarity to introductory, transistional adverbs; e.g., nevertheless, do not arrive early. I think the pause is much more pronounced in those cases.

Here is all of the statement on this usage in Webster's Third New International Dictionary.
4 either adv 1: at all : LIKEWISE, MOREOVER --- used for emphasis after a negative <they are the best available and are not expensive either> esp. one contradicting a previous affirmation <it's raining. It isn't either> or agreeing with a previous negative statement <I didn't see it. Nor I either> or supplementing one <you'll not go far in life and you won't be happy either -- W. J. Reilly>
um...no, either is never and can never be a pronoun. everything else you say makes sense, though.
Either can be an indenfinite pronoun — along with such as words as "all" and "none."
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