smallest 4" design

[alan m]

hi there.
i was thinking of making a small seperater for a really small dust collecter(the small portable type you could put under your arm) i havent bought it yet.
i want to use it for demo work on site ,chasing walls,cutting tiles etc.
i will put the pipe out the window and blow the dust outside and away.
but i dont want stones or bits of tiles going out or hitting the impeller.
i am not worried about the dust  really but the bigger stuff.
how small do you think i could make the seperater 
i was thinking of putting it on top of a  bucket or small barrel .

thanks alan