I compiled my story as an epub3, but the page width of the outputted file is too narrow. I need to widen it. Unfortunately, “Edit Format” in options is grayed out. How do I fix this?
Many thanks!
Hi Obie. Welcome to the forum. 
I’m afraid you are gonna have to provide more details, as your question is a tad confusing:
1- Epubs don’t have page settings. There is no fixed page, and therefor no page size, in an ebook.
2- “Edit format” in the options… what’s that ? (If you meant the default formatting, that has nothing to do with it.)
If you meant this:
…double-click the format instead, and duplicate it first as prompted.
You’ll then be able to edit the duplicate, but as I said, epubs don’t have pages.
The only thing I can think of that could cause this is if you’d have used a right indent. But given that you’ve yet to access the compile format, I can’t think of how that’d be possible. Unless a) you’ve set your default formatting as such and compiled “as-is”, or b) you used a style pretty much all over your body text, with a right indent set in it. (?)
In the editor, select a small chunk of your body text that you know for certain came out wrong (keep your text selection within the boundaries of a single paragraph – one word is enough as your selection) , and look at:
Do you get a value other than zero ? (Zero is what you want.)
(And you also don’t want to use styles all over the body text.)
If your text displays something other than “no style” here:
(Format bar above the editor.)
we’ll then help you with fixing that.
At the bottom left corner of the Compile Overview screen is an options button shaped like a cog. If you click on that, a number of options appear, including, at the very top, an “Edit Format” option. That’s what I was referring to. It helped me adjust a couple of things before I compiled a PDF. I just presumed it would do likewise with an epub3. I guess I presumed wrong.
I’ll check
Thanks for your response.
The problem stemmed from the paragraph indents. All fixed now!
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E-books do have margins, and too high a value will result in a small column or ‘page’. So check the @page {margin: ??px;} CSS style. You can set it to zero, because e-readers also govern margins. (So, check your e-reader setting for margins as well) 
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You can edit the xhtml and CSS stylesheet in an e-book editor, like Sigil.