I did make this blanket using the US 50 needles and 4 skeins of yarn. Since you are only using 2 skeins and size 15 needles there will be a big size difference. Using 4 threads made 1 stitch = about 1".
I agree with the previous poster - keep knitting until you get to the right dimensions. Even though the pattern only calls for 1 skein of yarn for each color you will probably need more.
The speed stix were tough to work with. My biggest complaint was that there was not enough room on the needles to fit all the stitches when the blanket got to it's widest point. One of the reasons I liked them is because it really did knit up quickly, but, not sure if I'll do it again. I want to make this again - not as baby blanket - and I bought size 19 needles so I wouldn't have to use the speed sitx!
Full Size Baby Afghan
Afghan Stitch Detail
[size=2]Edited only to change the source of the photos[/size]