I am making a hat with size 8 needles, the only wooden ones in the store that I could find were 26". I know that 16" would be better for a hat, but is it alright to use 26"? What is the difference (I am a beginner!)? Thanks!
Circular needle length
wrap the 26" needle around your head…
basically the stitches that you need for the hat will not fit around a 26" circ and if you try to cast on enough stitches to fit around the 26" it will fall down on your head.
of course that is assuming that you are knitting in the round. if you are knitting flat it doesn’t matter at all! 
Are you making a hat that is 24 or 26 inches around? If you use a long needle the stitches you need won’t fit all the way around. Most hats are maximum 23 inches around, and that is when the fabric is stretched.