I went to Hobby Lobby today and bought my first DPNs…
apparently the wrong size. I got size 7s, and it seems like there is nothing I can do with that size.
My question is, does it REALLY matter if the pattern says size 6 needles or size 3 needles? If it does matter, what can I do about this situation? Thanks 
ya, it kinda does matter…if you knit with too big of needles, your guage will be off…the yarn will be too loose, if you knit with too small, the yarn will be tight. Some yarns won’t work well on smaller needles because it is too chunky.
if size matters, guage, and see how your stitches are coming out to the reccomended size, your knitting might be tighter or looser than what is reccomended. 
hth’s
You can make a swatch with the yarn and needles you have and use the numbers from that to make a hat using this formula.
Otherwise, though, you need to be able to get the same stitches per inch (gauge) that a pattern calls for. Otherwise, like Heatherfeather said, you’re end result is going to be much bigger or smaller than the pattern says it will be.
Like this pattern,
http://www.knittingonthenet.com/patterns/socksthreesizes.htm
Can I just use my 7s and follow the pattern?
If you are someone who happens to knit exactly on gauge with the pattern, and you use a 7 needle and the yarn called for, your socks will be too big.
If you are a tighter knitter, and use a 7 you could be right on. I always seem to have to go up a needle size.
Start the sock, and after you’ve gotten a few inches done, check to see if you have about five stitches per inch. If you do, you’re all set.
If you have more than that, your sock will be smaller than the pattern says. If you have fewer than that, your sock will be bigger than the pattern says it will be. Then, maybe you could make a smaller or larger size.
You could knit up a gauge swatch, but in this case, I think starting the sock on the dpns would be the best gauge swatch of all.