Imovie 10.0 requiring too much space

Imovie 10.0 looks nice and I deleted older version after downlading new version.

now while working I am struggling for space. as I have only 64 GB on internal mac air 2011 mid summer edition. I am keeping 5gb free still computer space becomes full and computer hangs.

1> How do I reduce space required for the imovie. I am attaching 1 TB externel hard drive through thunderbolt connection.

2> How do I download older version . I had no such problem with older version.

3> Can I expand to higher space in hard disk. how does it cost?

thanks in advance.