Can I edit the size of my photo when setting up an email signature?
Not sure what you mean by "photo screen?" Do you mean your Photo Stream?
In the Photos app there are several ways to view the photos. Tap on Albums. Your Photo Stream is one Album but there may be others including the Camera Roll, albums made on the iPad with photos from the Camera Roll, and albums of photos copied to your iPhone by syncing with your computer.
To remove albums from the Camera Roll, tap on the Camera Roll album to see the small thumbnails of the photos there. Tap on Select, tap on the photos to delete, then tap on the trashcan. The photos will then be moved to another album called Recently Deleted. They remain there for 30 days to allow you time to reconsider whether you really want to delete them or not. After 30 days they will automatically be deleted from your iPhone. But if you are sure you want the photos deleted from your iPhone delete them again from the Recently Deleted album.
Albums created on the iPad with photos from the Camera Roll can be deleted in the same way.
To delete albums copied to your iPhone by syncing with your computer running iTunes are deleted by doing another sync. But before you click on Sync deselect the albums in iTunes. Then when you click on Apply or Sync the albums will be removed from your iPhone but they will remain on your computer.