If you have a needle near the same size, you can use one of each. The newer part may be a bit narrower/wider that the beginning, but it's a scarf--it doesn't have to fit.
Or you can start again with two different sized needles. If they are reasonably close in size it won't pose a problem. If one needle was really small, then using a larger needle might be difficult when you're knitting from the small needle.
"Trust the pattern!"
Time flies like an arrow, but fruit flies like a banana.
Hop aboard while we follow the cloud-covered van of knitting harmony, man!! Woooo!! --Hedgehog
IngridKH on Ravelry