How big in real life?

You cant get an exact measurement because it all depends on the individual stone but...

The chart I found said a .25 is 5mmX3mm for an ideal radiant or emerald.
.50 is 6mmX4mm

so for a .4ish stone I would guess somewhere in the upper 5ish mm X upper 3ish mm.

Not very exact but hope that helps.

Chart for reference:
http://www.ajediam.com/Emerald_diamond_sizes.html

There's a thread over in "Show Me the Bling" called "Under 1 Carat Diamonds" - if you're trying to get a visual sense of how it will look IRL, I'd recommend going over there and taking a gander.

ETA: Annnnnnnnnnd ... link!http://www.pricescope.com/forum/show-me-the-ring/under-1ct-diamonds-t25028.html