you can always cut for short. Maybe not the sleeves.
Anyhow its easier to cut than to add. BTW, for big and tall theres: regular clothes tall, very tall from big and tall. If you fall between the two catagories your screwed.
Im in my 20's and my torso will look short if I tuck in.
I don't think dress shirts, like legit dress shirts, should be worn un-tucked. if someone wants to add 'personality' or 'flare' to their dress shirt in my opinion it best be done through the way you wear your cuffs and what you do with your buttons.
Agree - some shirts just can't be worn untucked and others are specifically designed for it, e.g. Folk shirts, which sometimes have a detail of some kind at the base of the garment, clearly meant to be on display. I have one or two dress shirts and they'd look like nightgowns if I wore them untucked.