A common workflow involves importing an image, marking it up, and exporting again at the same size as the original. Images and PDFs will automatically be imported at their original scale, and exported at the same size if you choose "Original Drawing" as your export region. Make sure your Output details match the intended size, Concepts has options for exporting at various file sizes (see device-based export sizes here).
 
If you want to crop or resize the import to a specific size:
  1. Open the Settings menu (tap the gear icon on the status bar) and set your artboard size to the size of the image you are going to import in pixel units.
  2. Import the image you want to mark up and with the image selected, scale and position it to match the artboard size.
  3. Once you're done working with your image, open the export screen and choose "Custom Size" so that the export reflects the size of the paper.
  4. Your exported image will match the pixel dimensions of the imported image.