My wife's vagina (like many I'm sure) - is quite short to start out. In fact I can touch her cervix with my middle finger - and I have small hands - that's just 3.5".
But as she becomes more aroused, her vagina elongates and since I'm about 7" her body has learned to accommodate me - and any dildo that length. However we have a couple of longer dildos - and when highly aroused she can get to 8 or maybe 8.5 inches. She can take most - but not all of the Vixskin Outlaw - which is 9" long.
When we get into using the longer toys - I let her be the guide. I'll press forward being sure not to go too far - then as she is ready, she press against me letting me know to go a little deeper.
Soft toys like the Vixskin are quite a bit safer. If your using glass or metal - extreme caution has to be exercised.