Direct link to pattern
. The yarn seems to be a worsted or heavy worsted, so going up in yarn weight and needle size won't work. It helps if you knit loosely though, so going to a size 10 and 10½ might help.
So the back sts measure about 20". After the ribbing, you can increase in increments of 4 sts, to get to half the bust size you want. Say you need about 44" total, half is 22" which is about 2" more so you need to add either 8 or 12 sts, depending on what your gauge is. Just follow the armhole shaping instructions, they won't really change a lot.
For the Fronts, add 4 or 8 sts, whatever will work with the back stitches. That should help. I'd recommend you start with one of the fronts first. It's smaller than the back, so after 6 or so inches and you find that it's too small or too big, you won't have spent as much time on it.