These are the 4 image sizes that Modules Theme uses:
- Squared Thumb: 700px x 700px
- Medium Thumb: 700px x 507px
- Medium-Full Thumb: 700px x auto height (this size respects the image original ratio)
- Full Thumb: 1400px x auto height (this size respects the image original ratio)
This means that every time you upload an image to your site, in addition to WordPress default thumbs (Thumbnail, Medium and Large), the theme will create these 4 versions of that image by cropping and resizing it. And then use the different versions across your site, according to the design.
If the images you upload are smaller than that, the theme won’t be able to crop and resize accordingly.
So, if you want to make sure your images will always look good, you should upload images that are at least 1400 x 700. And the theme will take care of cropping and resizing for the 4 sizes.
If you pretend to support retina devices, then it’s 2x that size (2800 x 1400).
Now, there are places in your theme when images are used as backgrounds and not as < img > HTML elements. That means those images will always cover an area, no matter the area’s size and dimensions. Like when you use an image as the background of a module, or in a Slider module. In those cases, having an image that’s at least 1400 x 800 is the best way to assure those areas will be nicely covered.
If you know an image will only be used on a Services module, for example, and never as a big background or a full-width image, then you’ll be safe by uploading an image that’s at least 700 x 507 (the Medium Thumb).
In short, sometimes covering the size of the 3 first thumbs will be enough, but if you don’t want to worry about that at all, then uploading 1400 x 800 sized images (or twice that to support retina devices), will be the safest way to go.
If you have doubts about a specific size, like what size should the image on a specific module have, jump into Module’s forum and open a topic with your question.