Early on it may be unclear what the optimal size is to upload. You don't want to miss out on potential sales because the resolution is too low, but you also don't want to waste time uploading pixels that won't be needed.
Unfortunately, there isn't a default answer to this. The right image size depends on your specific genre and audience. If you have been selling photos for a while, you probably already know what size your audience generally want. You can simply refer to our printing size guide and pre-size your images accordingly.
If you don't yet have an idea, you'll need to experiment a little...
If your event is relatively small, say 200 images, or you have a bit of time, we suggest uploading images that are about 4MB to 6MB each.
If you have more than 200 images, or need to get photos up in a rush, I would recommend pre-sizing your images to 2500 x 1666 pixels.
The idea is to keep an eye on the customer feedback, and adjust with each event until you've hit the sweet spot between definition and upload time. Please also note these important things to keep in mind regarding image quality.