Disclaimer: These ideas are theoretical, I haven't tried them, and they might result in you needing more than one skein of your selected yarn.
As soon as I designed my Top-Down Alpaca Mitts (formerly known as: Top Down Fingerless Mitts, Taj Mahal Mitts, and Ambrosia Mitts, phew!), before I even submitted them for 101 Luxury One-Skein Wonders, I received a question about making them in a larger size.
No rest for the wicked?
I was showing them off at the yarn store and one of the other staff members tried them on, observed they fit her, but said a little bigger would be better. So I've been thinking about these ideas for a while, but in a very lazy manner.
Making Them Longer
There was a method to my madness when I started the mitts at the finger end rather than the cuff. I thought they might be easier to resize. And, based on the principle that the leg of a toe-up sock can be knit until you run out of yarn, I thought the cuff could easily be made longer from this direction, i.e., you can try them on as you go and get a better idea of where the cuff is hitting because the thumb hole provides an anchor.
So to add length on the fingers, I would suggest working another pattern repeat of the lace. When I made the gloves for myself, I was aiming for the cast-on edge to sit at my first set of finger joints. They might end up shorter on you if your hands are bigger or your fingers are longer.
When you try them on, position the cast-on where you want it to land, work until the needles sit in the crook of your thumb, then make your thumb hole.
Adding length to the wrist end is easier: either work another lace repeat, switch to ribbing, or keep going with the smocking.
And, hey, you don't have to work the ruffle if you don't want, I won't mind. :-) Not that anyone has complained.
To Make Them Wider
Now, I'm a small person, so I have small hands (which is a good thing, I like being proportional), but as the mitts have a lot of ribbing they have a good amount of stretch to them.
Still, if you want to make them wider, I think the easiest thing to do would be to add more ribbing to the side or palm.
You'll have to be a brave kniter to do this, the pattern would serve as more of a guide in this situation.
Keep in mind the purl columns on either side of the lace and cable pattern on the back of the hand are part of the pattern and you shouldn't mess with them. And remember to keep purls on either side of the palm cable so it stands out.
If you do add knit and purl stitches to the sides, you might want to change the rib frequency to maintain a balanced look.
To aim the thumb hole in this situation, try the mitt on and keep the lace pattern centered on the back of your hand, then take a look at your anatomy, I think you'll figure it out.
(I would think you could still follow the thumb gusset decreases in the pattern. You would only have to change that if you make the thumb hole larger.)
Another thing to keep in mind is the smocking is on a multiple of 8, or is it 5?, oh, I can't remember. See, I told you my ideas were theoretical! Anyway, look at the pattern in the book, I have you decrease some stitches at the wrist to make the smocking work. If you add a bunch of stitches to the hand you'll want to decrease or increase as needed to make sure the smocking will fall out correctly.
I bet I haven't told you anything you hadn't thought of already. :-)