Literally, it is a shadow of someone standing or walking with spread legs. It works as a literal description of Emily's father's stance/gait. However, think of its symbolic meaning too: behind Emily there is (to translate the phrase) a sprawling shadow. It spreads out, filling the space, and her house and life is very much filled with darkness. Her father's black mark hangs over her, spraddling her life.

You might be finding Faulkner's writing difficult because he does tend to use the southern dialect. Let's look at the word "spraddled" in context:

The Griersons held themselves a little too high for what they really were. None of the young men were quite good enough to Miss Emily and such. We had long thought of them as a tableau: Miss Emily a slender figure in white in the background, her father a spraddled silhouette in the foreground, his back to her and clutching a horsewhip, the two of them framed by the backflung front door.

Imagine the scene: Emily is standing behind her father, who is sitting in a chair on the front porch, whip in hand. Think of "spraddle" as a combination of "sprawl" and "straddle." To straddle a chair means to turn it around backwards and sit with the back facing you. To sprawl means to spread out. So we can see Emily's father straddling the chair, but sprawled out--not an imposing, stern, erect posture, but a relaxed pose.

I hope this helps!