With the size 15s, I'd make a guage swatch using the pattern to measure how many inches per X amount of pattern repeats you get. (You can choose how many pattern repeats you want to test.) Then I would note out how many sts are in the pattern repeat. Calculate your desired size of the blanket to determine how many sts you should cast using the size 15s.
There is still quite a difference between size 15 and 35, so it might still end up looking different from the pattern picture. But I would worry too much about that because you'll be able to see the difference after you finish the swatch, so you can determine right then if you like the pattern on size 15.
Hope that helps!