1. I got these sentences from Ready References for your Pocket Testament and am wondering why there seem to be different size dashes for the three sentences below.

Present Christ-not yourself, nor your church.

Hold the conversation to the main point-acceptance of Christ, and, above all, do not be drawn into argument.

Forget self -- live for others.

2. What is the difference between the two? I see very little differences, if not at all, between the two.

I am doing the best I can.

I am doing as best as I can.

Should the underlined part be "difference" or "differences"?
Read about the m-dash here:
Believer, very few people aside from those who habitually proofread and edit will notice whether you have used an em-dash or an en-dash.

And few people will bother to insert an em-dash (—) or an en-dash (–) in running text outside of real word processing programs. Since I follow the Chicago Manual of Style (mostly — I do leave spaces on either side of my em-dashes), if I'm going to the trouble of inserting actual dashes instead of using the hyphen (or double hyphen), I will use an em and not an en. The article says to use the ens if you have short-width columns, which is usually not a problem for most writers of normal prose.

I can see why people want to use en-dashes. The em-dash above looks absurdly long. But if you're going to go to the trouble of learning when to use an em and when to use an en, my advice would be to ignore that sentence abotu being able to use the ens instead of ems.
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Thank you, MH.

How can you differentiate in terms of their usage when the online source Wikipedia has noted like this?

"Like em dashes, en dashes can be used instead of colons, or pairs of commas that mark off a nested clause or phrase."
 BarbaraPA's reply was promoted to an answer.
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