Untidy writings are actually a reflection of a part of your
character. As such it is very difficult to change. You see your
hands is merely a tool of expression of your thoughts when u are
writing. So learn to be a more organised person and you will surely
write neater.
As for why u write small, I am guessing you are person who can't
and doesn't like to express yourself. If that is true, then perhaps
express yourself more and tell people more and u may see your
writings growing bigger!