Artboards can be exported in several ways: a single artboard as an individual image, all artboards concatenated in a single image or multiple artboards as separate images.
If you select an artboard before proceeding to export, the selected artboard will automatically be selected in the Export dialog.
If you choose to export the entire document, all artboards will be concatenated in a single exported image. The resulting image dimensions will be based on the smallest possible size relative to each artboard's size and position on the pasteboard. In other words, the smaller the artboard sizes and the closer they are together, the smaller the overall image's dimensions.
You can also simultaneously export all artboards as separate images. The Export Persona is used to create slices (export areas) automatically for individual artboards.