Size of page reduced

Since I got such an unbelievably quick response to my question about widgets, I’ll have a go with another question.

While I was trying different things to fix my sidebar before I came here to this forum, something I did suddenly reduced the size of the page to about 50% of what it was originally.

I have one other WP site that I set up about a month ago, and this never happened.

How can I get the full page back? Will it just automatically lengthen as I add content?