I style.css file of your theme, you have two places that
you can play with.

Note:
Before you do anything, save the file first.
Using FTP, transfer the file style.css onto your computer.


pre { overflow: auto;
width: 470px;
}

That is most probably your sidebar width.

.post img {
max-width: 680px;
}
This is probably your content area.

You can change the width of either one. But make
sure when you increase one like
max-width: 680px; then you lower the number 470px by the
same amount.

Try it. If doesn't work, re-transfer style.css from your
PC to the server.

In any case, on either part, there is obviously a limit what you can do.

You can also resize the image and make it bigger to what you want.