I'm not familiar with Logic or Space Designer, but the current issue of Future Music has an article on psychoacoustics with a couple of general pointers on this.

1) Introducing a delay (suggests 1ms for every foot distance)
2) Roll off the high frequencies slightly to match real-life dispersal
3) Echoes and reverb, paying particular attention to early reflections and decay time

(When using reverb you'd usually have a dry/wet control to balance the original signal with the effect.)

Set up a pre fader auxiliary or effects send (essentially the same thing but different DAWs name them differently). In the auxiliary/effects return set up your reverb plugin with a 100% wet ratio. You decide which verb to use. I don't know how big you imagine the room to be. You control the amount of signal gets sent to the verb via the aux/effects send. Control the amount of reverb with the fader on the aux/effects return. If you play with this you can really fine tune the distance. Doing it this way also allows you the freedom to bring the instrument closer by raising the fader on the main channel without affecting the amount of signal going to the verb. You can set the verb to be very wet and still bring the instrument closer but it "remains in the same room".

Of course this is just the way I do things. No techo jargon involved. No calculations as to how many feet away. I just do things by ear and when it sounds good, it's good.

Thanks for the excellent advice. I've been fooling around with it, and I'm struggling with the question of whether or not I include the original signal at all.

...yes, otherwise you're just hearing the echo, and that ain't natural.

I'm imagining a trumpet at 20 feet, and exactly what I'd hear if I were standing there... the sound directly from the trumpet would hit me at almost the same time as some of the early reverb - if I include the original dry signal, am I not defeating that distance concept? It's a dry direct sample.

You don't have to include the dry signal. It's your vision. Do what you must to achieve it.

Totally agree - if it sounds right, it is right. But I just meant to explain that, usually, the reverb effect is mixed with the original signal rather than used in isolation. As you say, you would be hit by the direct signal and the early reverb at almost the same time. Not at exactly the same time though. And the minute time difference is part of what creates the impression of space.