my buddies and i built one last summer. We did not plan really at all we just kinda started building, so i would say make a plan or research good designs.
ours was like 50ft by 70ft give or take a few
also choose a good place because we built ours right in the entrance to a field that would later be used for dirt jump building so we had to run over our pump track with a tractor daily, SO DONT DO  THAT.
also dont let cows in, we did that too.
AS far as maintenance, if you ride it for 2-3 hours with about 3 other guys it will need to be manicured. When ours was half built we just had 3 rollers into a big 180 degree berm and once we got a feel for the berm it got destroyed so, just manicure it after every session.