Shield isn't long, it's tall. And is made for CQC. Also she's still taller than the shield and strong enough to move it with ease. Posture for holding it is extremely flexible, entirely up to the extension and bend of the arm. None of this applies to the guns.
@Pen3: You can hug a shield close to your body and it'll still serve its purpose (blocking incoming blows)
Whereas if the enemy get too close, the SCAR can't easily just aim at their weak spots because it'll hit the other body parts instead.
If you go look at page 5, you can see it's as long/longer than his arm length, whereas the P90 even with its silencer added is probably close to 2/3rd or half that length.
@pen3 the main difference is that you need to point the shooty end at enemies with a firearm, while the shield has no such requirement and you actually *want* it to touch the enemy.