Settings → Storage shows how much of your plan’s file quota you are using.
Typical limits: 1 GB Free, 2 GB Spark, 5 GB Starter, 25 GB Growth.
Counts toward storage:
Campaign images
Organization logo
Widget canvas images
Widget icons
Donor photos
When you near the limit, uploads can fail. Remove unused campaign images or donor photos, or upgrade the plan. Storage is not the same thing as the campaign/widget count cap.