Telescope eyepieces with a focal length of 5mm or less are ultra high magnification eyepieces. As a rule of thumb, we recommend 25x magnification/inch of aperture for planetary astronomy, and 50x magnification/inch of aperture for deep sky astronomy. If you need help calculating field of view, or magnification, reach out. We're always happy to help. We also have an excellent guide on how to choose eyepieces here.
