You can find the dimensions of images for such things as the sliders on the main index pages, images shown by the modules and widgets by going to the Style Guide for your site. You can view the gallery by entering your website's URL followed by /style-guide. For example, for fallhaven.org the style guide would be found at fallhaven.org/style-guide.
If the dimensions are not listed, please send a support ticket in and the information can be added.