I had a similar problem, with vertical scrollbars appearing when I set a maxWidth in the Fancybox options.
To get around the problem I had to set
.fancybox-inner {
overflow: hidden !important;
}
and set a fixed width CSS rule on the Fancybox content rather than specifying a maxWidth in the Fancybox options. If I did the latter, Fancybox's calculated height for the content was slightly too small - probably hinting at why it was putting in scrollbars in the first place.