in vitro transcribing a long RNA molecule?

[Z.L. K]

I will be doing an experiment for making the RNA molecules with the
size up to 8100 bp in length. Since this will be my very first
experiment handling RNA molecule, i would like to learn if there is
any perticular thing I need to pay extra attention to, also if there
is any tips you would like to share with me in order to successfully
make such long an RNA molecule. Thanks a lot!
ZL