And maybe it is. But I have returned to my earlier, uninformed strategy: I choose whatever looks good.
That said, slender stalks can burn easily, turning what you hoped would be a nicely charred vegetable into an asparagus crisp. Fat shoots tend to require so much time on the grill to reach tenderness that their outsides can turn soft. Medium-size asparagus, I've found, takes well to charring while remaining simultaneously crisp and tender.
A bigger factor than size is freshness. If the asparagus at hand is limp or its spear ends flake easily or any part of the stalk is wrinkled, I change dinner plans and choose a different vegetable.
Depending on my mood, I might get out the vegetable peeler. Peeling the stalk reveals a pretty, pale green that can seem almost translucent. I cannot vouch for a significant difference in taste (although I do think the flavor becomes less "field" and more "stream," if that makes sense). But sometimes I just prefer that clean, stripped look.
The versatility of asparagus is yet another of its many virtues. I will never forget an asparagus risotto that my wife and I enjoyed in northern Italy, at once rich, light and bursting with the flavor of springtime. Grilling the asparagus enhanced my attempt to replicate the dish at home.
I go back and forth about cooking asparagus in a grill basket. Generally, I don't, because I feel that grilling directly on the grate gives the stalks a uniform char. But sometimes I do, perhaps because I may be in a pinch for dinner and I don't want to risk any casualties (spears falling into the fire).
I also love an asparagus soup as a starter to a meal that moves on to other springtime glories, such as lamb. In addition to grilling the stalks, I briefly smoke them to lend the soup a beguiling flavor note that adds complexity to the sprightly springtime taste.