You should be able to do this with three commands:
Keys: [SPACE_DOWN]
Delay: 5 seconds
Keys: [SPACE_UP]
That won't send a million spaces, like holding the physical spacebar key for 5 seconds would do, but it would report that the spacebar was down if any programs were checking for it.
I'd have to echo gemesigo's question of what you're trying to accomplish by doing this. It seems like there may be a better way to do whatever you're trying to do.
-- Jeff