You did check out Ribs 101 didn't you???
Although it doesn't go into "what's best" that's a long answer.
Ribs 101 All sizes are best. Some just better than others.
I think you won't have as much choice, you'll probably have to go with what you can get, unless you can go to the butcher.
Close Hook, but no cigar this time, don't know that I've seen 3 1/2 and down backs before but HEY I haven't seen it all.
For spares, size depends on type. If you get trimmed spares, called St. Louis trimmed/style, go for about 2.5 lbs per slab. Untrimmed spares, you'll see about the size Hook Mentions. You'll see some over 4, and we call the Bronto Spares (like the beginning of the Flintstones Cartoon -- they're really big, and I don't recommend them.
For loin back/baby backs, got for 2lb and down (called 2 and downs). Lots of people like 1.75, I've had some good one's lately, big ones, about 2.25 almost 2.5, but never bigger.
Depends on price too. I'll do BBacks in my contests cause I'm getting better looking Backs than Spares. But the problem is price. Usually backs are about twice the price.
And we haven't even covered, it's not the size that matters but how they look. When you look at them, look for meat, not a lot of fat. Look for shiners if you can, those are bones that were trimmed so close they actually can be seen.
For practice, I recommend spares. They're cheaper and if you go with untrimmed, not St. Louis, you get a lot more meat.
That's my quick answer. I'll be sure to add info when I do ribs 101 update. And I've got to update those photos, I've got better pictures of ribs.
Smokin'