I've just created this riddle for you, hope you like it! :)
I can be the first like I can be the last, but I'm usually in the middle between.
I'm not the only one here, we're many.
I'm just existing in daily life, but I'm not living.
My name is composed by 6 letters.
I can be big like I can be small.
A group of me and those similar to me can be composed by at maximum 26 different types.
So, now try to guess who I am