5mm seems awfully short. Does it work as well as the 8mm or a short insulin syringe which is about 12mm? What is too short?
GEO3610
There are 4mm pen needles out there too. They work, and seem to be what are recommended now, but I like them a bit longer, because it seems to be easier to find the thicker needles in longer lengths. Really fine needles (31 and 32 gauge) are a bother if you need large injections, and they bend too easily.
Try them and see what you think is probably the best advice.