I could use your assistance to see if this is possible, and if so, what appropriate stats for it might be.

Basically I'd like a physical barrier spell that when cast, you decide to narrow its range of effect, while in turn increasing its effectiveness against that range of effects.

So you can cast it as normal, 1 armor/structure rating per hit. Or you can fiddle with its range of effect before casting.

Fiddling with the effect can result in a barrier that's better versus small fast projectiles like bullets and arrows and flechettes, but consequently does nothing for 'thrown' objects or larger. You could make a barrier suitable for affecting big things like cars, maybe trolls, but anything smaller would be unimpeded. Would such an effect be too broad/powerful? Could you say make it like the Dune personal shields (the slow blade pierces, etc)

Should I keep the 1 armor/structure per hit mechanic and instead make this modified barrier 'easier to cast' as in less-drain, or some other approach?

Though SR5 spell creation rules are not yet available, in previous editions, you could make specific target spells, that only affected certain things. Such as Wreck Gun, and such. So I see no reason you couldn't make a "Physical barrier: Bullets" that only stops bullets, but not the knife, car, or fist coming at you. (All three. Yes, you're having a bad day)

Would "Grenade, Rocket, Mortar" be too broad?

Explosion or Concussive Force maybe?

I'd go with Sendaz's solution. Of course, if the grenade penetrates the barrier first....

Well that is the risk you run with using a filtered (specific defense) magic.

For this case you might want to tweak the Armor spell instead of the Barrier, that way the protection is unlikely to be circumnavigated (penetrating prior to exploding) unless they actually hit you physically with that grenade first.

Bullet Barrier was an old favorite in earlier editions. Also a favorite for anchoring, tied to a detection spell.