I am presuming you are using the Easy Fancybox plugin, or some equivalent plugin. Your problem could be solved by applying custom css after Fancybox has loaded, as suggested by the plugin author here. In essence, you can use media-queries to specify the size of the Fancybox to be different on various devices. See for example this article as an introduction on using media-queries.
Also, per the Easy Fancybox FAQ, a fix could be:
The lightbox does not look good on mobile devices. What can I do about that?
The FancyBox 1.3.4 script that is used in thsi plugin was not
developed with mobile devices in mind. The only way around this issue
is currently to disable FancyBox for small screen sizes. You can do
this by adding a text widget in your sidebar with the following code
snippet.
<script type="text/javascript">
if(window.innerWidth < 500 || window.innerHeight < 500 || window.outerWidth < 500 || window.outerHeight < 500 ) {
jQuery(document).off('ready gform_post_render', easy_fancybox_handler);
};
</script>
Tweak the value 500 to target other screen sizes.
Hope this helps, best of luck!