No the vagina muscles don't permanently stretch even with child birth so a dildo won't cause permanent enlargement. What building up to a larger size will do is allow a woman to learn how to relax the muscles to allow for easier penetration, the same thing as when you are learning to relax in preparation for anal sex. The anal as well as vaginal muscles can be strengthened and made tighter with work, that's what kegel exercises do. Like any muscle if you don't work out it will get flabby and loose. Generally speaking only repeated child birth without waiting at least three years between children will permantly stretch vaginal muscles and that is due to the changes caused by hormones not any stretching of the muscles. After three children I am tight enough that I need to be told to relax to have a speculum used during my yearly, so it's a wives tale that childbirth ruins your tight little puss.

Your wife might find larger dildos to be uncomfortable since she is used to your size but they shouldn't cause any noticable difference to you.

On a side note, oxytocin can cause a softening of the vaginal muscles and is released when you run your palms over your skin or a partner's. Also nipple stimulation will cause a release of oxytonin...so stroking your partner, or stroking yourself and your nipples will help you relax and allow your vagina to stretch just a bit making larger dildos more comfortable. As a bonefide size queen I can attest to the effectiveness of this type of "foreplay". less