I'm not a plus-size, but I've had issues with button-down blouses for most of my adult life. I can rarely, if ever, just buy one off the rack and have it look good so it's usually something I have to make (goshdarnit). Why? Well, let's see, I'm taller than the wee 5'5" clothing average, and although I'm of an average bust size I tend to be a bit fuller around the rib cage and waist than the standard measurements. (One magazine called my body type the "chili pepper" and I think that sums it up nicely!) The result? 99% of blouses that I try on have horrid gaposis, are often too short, and are, therefore, unwearable. Now, you could say, why not buy a bigger size? I could, but often the shoulder area is too big and that lovely tailored blouse turns way frumpy on me. So I have to make them, and here's the plan I follow:
First of all, you have to know your figure and what fit issues always come up with commercial patterns. For me, I usually just automatically let out the side seams a bit (especially if the seams are curved in at the waist) and, if there are darts, either make them smaller or remove them altogether. I always have to do this for anything slightly fitted. It's just automatic. You want enough ease around the rib cage, bust, and waist area that you don't see any pulling. I also will often add an inch or so to the length because I also don't want to be tugging at the darn blouse all day.
But the secret, once I've got the torso modified, is in the button placement. You must make sure that there is a button right at the apex (yup, that's the nipple line) and then refigure the placement of the rest of the buttons. I use that handy little tool there for this. Take a look at the picture of our lovely 1950's lady above to get an idea of what I mean, see where the apex button placement is? Do this, and the blouse will fit much, much, much better! I will also often add a button or two if I think they are spaced too far apart, especially on anything that hugs the body or that I've added length to.
Now that's just me and how I work out my own fit issues but the button placement is universal. The next time you go blouse shopping, take a look at where the apex button is when you try on the blouse. And please, don't wear a sports bra when shopping for blouses, wear the bra you'll wear with it because that changes the apex placement too. :)
For fitting commerical patterns, I really recommend Sandra Betzina's book Fast Fit. There was also a recent article on BurdaStyle about fitting clothing that can be found here. It includes lots of links and book reviews of other books on fitting patterns.