I am probably not the guy to ask about type etc. I don't consciously pay much attention to it. I only seem to pick up on the abnormal or extremes. I believe that most of what creates and regulates the desirable attributes, deep rolling, speed, return to the kit, teamwork ... come from within the bird and are for the most part not visible. Heart, character, mental motor, spin engine ... lots of names for the things that make the difference in these birds from being ordinary and exceptional. I think the mental factor is far more influential than the physical.
I do accept there are some body types, shapes and sizes that can generate more propulsion in the rotation and be able to turn over faster than others. I have heard guys talk about drag slowing the spin down, but I think velocity of rotation is more about how well they propel themselves or accelerate the revolution than it is about drag or resistance. I don't think type or size has anything to do with depth or return, except that they need to have some athleticism to perform. I believe length or cast, and keel shape and length mostly relates to style. The center of gravity around which the bird rotates creates a different illusion based on distribution of mass. The feathering just fills in the voids to complete the illusion of a shape be it ball or doughnut or in my birds case a wing shot goose. I like some cast in a bird, but I don't think it has anything to do with the depth, frequency or ability to kit back up, just the illusion.
It is just my personal preference (but is based from my experiences), I don't like short keeled, cobby bodied stubby tail built birds. I have seen and flown some that could spin pretty well. However, most had a rough or violent form/style/appearance when rolling, you can even hear them beating the air sometimes. I prefer birds that show perfect balance, clean and smooth, liquid silver in the roll. Also, little birds simply don't survive well here. I fly all winter and it can be cold. Lately it has been around -15 overnights with highs near 20 and the small birds cannot hold their weight. Where I live, we get a fair amount of wind through the spring and early summer and again little birds don't fare the wind as well as those with a little more muscle mass. I don't like coarse, big, lanky birds either. I consider mine to be a medium size and build. Some judges that have been here tell me mine are big, they may be. I think birds need some muscle mass capacity to be able to perform the duration of time they're up. Tiny birds and the shallow keeled narrow built ones seem to tire out (if they're performing) more readily than a bird with some meat on it that can hold gas in the tank. - Jon Farr