In terms of dilator size and penises, if you can comfortably dilate with the largest stent in your set, in practice you should be able to fit in penises bigger than the dilator (particularly for width). A dilator is completely hard like acrylic type material, whereas a penis has more spongy tissue. It's easier to have actual intercourse than dilate with the largest stents in my opinion.
The trans vagina does gape a bit following dilation/sex but it goes back to normal. In my opinion, the gaping entry is normal for a trans vagina, as it has some minor differences to a cis vagina. You can always have the commissure surgery done at the top and bottom of the labia to get the oval textbook vagina look.