Thank you for sending that to me. The problem is that you have an explicit column width set.
This is not exactly obvious, but there is a check box on the Format > Columns dialogue labeled "Equal column width". When you have a single column, it may be checked or unchecked, but it is greyed out. When it is checked it means, essentially, calculate the column width(s) based on other factors (page width, etc.). When it is unchecked it means use the stated width and calculate other settings based on this.
As you have this box unchecked, Word calculates a column width of 7.5" + left and right margins of 0.5" each, making a minimum page width of 9.5", and it won't let you change the page width to any lower value.
If you choose one of the multi-column presets in the Format > Column dialogue, Word will set this checkbox accordingly if it considers it relevant: click on "Two" and it will check it, click on "Left" and it will uncheck it. Word does not consider it relevant when you choose "One"; this is probably correct but it should not then grey it out and, frankly, I consider this a bug.
However it was that your column settings got changed in the first place, that check box got unchecked and when you changed it back to one column you did not check it (you couldn't!). The solution is to go back and change your Normal.dot: in the dialogue, first click on "Two" to force the checkbox to be checked, and then click on "One" to change back to one column, and Word will not change the checkbox. You will then have it set to using a single column that fits within the page, and you should be able to change your page width as before.
Enjoy,
Tony
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