craftgrrl | big stencil for pavement..

Has anyone ever made a stencil to be used on concrete or pavement? We're planning a shindig in the spring and I thought it would be fun to have stencils in the driveway to go with the theme of the party, so people can see it as they come up to the house.. (with water based paint so it will wash away)...

I figure it would require stiff plastic for the stencil, and an exacto knife... but has anyone made a person sized stencil? about 4-5 feet?

any and all suggestions welcome! Thx.

added : if you can suggest some 'street art' DIY sites that may have a tutorial or tips, that would be appreciated too! :)