You could set the width of the image in your html:
<img src="/images/image.png" width="200" height="260" />
Alternatively, you can set the image width to 100% in your css if the containing div is actually the correct dimensions. If neither of these fix the problem I will need to see your code.
PS. Photoshop's Save For Web feature is primarily for managing file types, color profiles, and meta data.
I had already tried your first suggestion it did not work. However I found
In photoshop resizing turno off the resample and set the pixels per inch to
72. Turn on the resample and then set the desired width. When I did this
and then Saved to Web, the resulting png exactly fit the box I had prepared
Thanks for your prompt response.