I agree with these guesses.
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This bird is from the older group, so is 20 or more weeks if that helps.The SLW appears to have a rose comb in the photo (pretty grainy on enlarge, but it looks like a rose comb).
Rose comb is a different kind of comb, and won't get big like a single comb will, so no, it is not large.
Now how old that bird is plays a big factor. You said no more than 20 weeks, with some closer to 14, so this would have to be a pullet almost point of lay, but maybe only about 14 weeks of age. With that amount of red in a rose comb, and blocky pattern, especially that much white block on the wing bows, I still say rooster for SLW.
Some hatchery birds have sloppy lacing, and if this is a bird nearly 20 weeks of age, you would expect a pullet to get a red comb by then as she would almost be laying...however Wyandottes do mature a bit more slowly, which means she might not be laying until nearly 24 weeks (6 months), so again we are back to red rose comb at 14 to 18ish weeks with blocky pattern.
My best guess is rooster maturing, but you may have to see if it lays or crows.
LofMc
That tells me to wait and see. While it is not as dark as it should be for a SLW pullet, with nice lacing, it is possible that could simply be from hatchery quality. The comb is red and fleshy, but that might be coming point of lay as it is not large for a rooster at 20 weeks and I'm not seeing saddle feathers.
BUT, Wyandottes mature more slowly and sometimes can be tricky to sex for the reasons above.
I'm leaning to roo, but post a follow up when you figure out which sex this bird is.
LofMc