i am knitting a jacket and about 1/3 of the way throught it i discovered i am using size 10/12 needles instead of size 8. it would be too difficult to unravel. what will happen if i continue with the size 10 1/2 needles? will it just be bigger or will it be out of shape?
Did you check your gauge? If you're a tight knitter and you're getting gauge, you'll be fine. If not, I think you're going to have a bit of a mess. It might just be bigger, but if there's a lot of shaping, all the ratios will be off, I think. Hopefully you knit tightly!
Does the pattern have a "schematics" section? Or does it tell you somewhere what the measurements should be for each piece of the sweater? If your measurements are on track, then all is fine. For example, if the sweater back piece should be 20 inches across and yours is 20 inches across, then all is well.
If not, then you may want to set the project down for a little while and consider your options. One option could be reading the book Knit Fix: Problem Solving for Knitters by Lisa Kartus. She has an entire section on radical fixes, including cutting and seaming a sweater that is too large.
The other thing to consider is that you may end up being short on yarn if it is bigger than the finished size in the pattern. If getting the same yarn and dyelot isn't an issue then you should be fine, otherwise you may want to calculate your yarn to see if you are going to run out.