As said in the comments, full-width or fixed-width has virtually zero impact on a screen-reader. The only thing a full-width page does is make the content longer visually (since it has more room to expand horizontally). From a screen reader perspective, widths don't really matter. It's just reading the text as if it were on one long line (obviously breaking it up in paragraphs and headings and such changes the way it's read).
That being said, going full-width can actually hurt accessibility b/c long horizontal lines of text are difficult to read. Accessibility is complex and there are a lot of different facets to it to consider.