It depends on how you intend to use the image. Each image page has sizing and dimensions listed. Here are a few additional guidelines across our image size options:
Small: good for mobile and most web uses including blogs, advertisements, and websites.
Medium: bigger web use (ex. full width background images).
Large: usable for almost all needs, such as full page prints or any type of web or digital usage.
When searching, you can use our filter tool to sort by size:
As a general rule of thumb, photos that are over 3000 x 2000 PX and 10" x 8" are better if you have larger use cases - like this photo for example, which is 6016 x 4016 PX and 20" x 13.3".
It is important to note that once enlarged and printed, DPI will not be as high (aka the resolution is compromised). If you are comfortable editing a photo to create noise/grain that would help give the photo a better resolution! This article does a pretty good job of explaining how it works: http://www.vsellis.com/understanding-dpi-resolution-and-print-vs-web-images/
Before you license a photo you can always download a comp so you can see what the photo will look like in your work before purchase. The "Get a Comp" feature is located bottom right hand corner of every photo on Twenty20 and you can download as many comps as you need!