RSG: Size of Universe
We could cheat. But we're going for realism - so let's not:
I'd like there to be about a million blob - that's the kind of number we could test in a reasonably short period of time at the start of each game - or after a long-range I-mort transmission. If each blob were 100 km across - they'd stretch like beads on a necklace in a ring about 100 million km in circumference with a radius of 15 million kilometers. The real asteroid belt has a radius of about 300 million km - and our version would be about three times closer to the Sun than Mercury! However, even with a maximum distance of 30 million kilometers, it's gonna take 100 seconds for light (or radio waves) to travel across it. That's a long time to wait - so perhaps we should go yet smaller.
It would actually be nice if there were blobs all around you - not just a thin string of them. If we had the belt have a 'cross-section' of about 10 blobs - then we'd be only 1.5 million kilometers from the star and now we'd have only 10 seconds to wait to transmit our brains across the disk.
If we're that close to the star then it's going to get pretty hot if the star is as big as our sun. So we need something smaller and cooler. The smallest 'M' class stars (red/orange-red) are less than half as bright as our sun (which is 'G' class). Gliese 876 would be a good candidate for us - it's "only" 15 lightyears away and has three extrasolar planets - one of which is within the 'habitable zone' and of a reasonable size for life. It's only about 1.2% as bright as the sun - and since intensity is proportional to the square of the distance, we could get about 10,000 times closer to it than the Earth is to the Sun and still be comfortable...so this ultra-tiny asteroid belt is a reasonable proposition.
It'll make for nice graphics because the star will be absolutely gigantic in the sky - and we get to have red illumination which will allow nearby asteroids to be clearly distinguishable from stars - and will let inhabited things like spacecraft (with nice blue rocket exhausts) and cabin/window lighting show up nicely.