QUESTIONS YOU NEED TO ANSWER:
Have you always had the condition where you tear at your perineum when you have intercourse? Before and after you gave birth?
If you have three adult children, I doubt your vagina is too small. Did you have an episiotomy where they sutured you tighter after your delivery than you were before you gave birth? That (and scar tissue with reduced flexibility) might be a reason for tearing.
Are you well lubricated? There are plenty of personal sex lubes on the market.
Is your husband's penis "tremendous" causing the tearing? I do not mean that as a joke.
Do you think your husband is good or bad in bed? That's also NOT a joke.
Do you have an orgasm when your husband orals you or masturbates you?
Do you have an orgasm when you masturbate?
Keep in mind that statistically, TWO THIRDS OF WOMEN DO NOT ORGASM FROM INTERCOURSE ALONE even when the sex feels great.
You should also speak to your gynecologist about tis specific problem you are having.
Answers to these questions can help us make suggestions to remedy your situation. I hope we can assist you in achieving your goals.