Re: If it was safe, practical and fully functioning....
Making it adjustable makes the question too easy. Since there is no downside to adjustable, there is no reason not to take that option.
I find the original question much more interesting. Women can always find out what it would be like with a different sized penis by using toys - available from small to ridiculous. I actually think its worth couples trying very large toys just to get past this. During a brief period when we were having frequent sex, my wife and I tried this. She brief enjoyed the novelty of very large toys, but quickly discovered that they really were just uncomfortable, and that normal human sizes were better.
The question is how many men want to permanently be larger, and if so, how large???? 6"? 8", 12". 24"???
What (unchangeable) size would women like their men to be?