I've just created this riddle for you, hope you like it! :)
Clue 1:
I can be the first like I can be the last, but I'm usually in the middle between.
Clue 2:
I'm not the only one here, we're many.
Clue 3:
I'm just existing in daily life, but I'm not living.
Clue 4:
My name is composed by 6 letters.
Clue 5:
I can be big like I can be small.
Clue 6:
A group of me and those similar to me can be composed by at maximum 26 different types.
Clue 7:
325-346
So, now try to guess who I am