Straw needles are thin/ fine, straight, long needles- another name for Milners. Often used for Beading because they are a consistent thickness, not ( fatter in the middle) .
I often use Sharps and of course the size depends on the weight of the thread I'm using. If I'm stitching with fine, silk thread I use a small, fine size, if stitching with a heavier thread I use a larger size. I like longer needles for some projects so do choose straw/Milners at times too.
Superior Threads is a good resource site with lots of information about different needles- types, sizes, uses, threads. A good place to learn about the differences.