I have the medium, which is useful for sidewalk chalk width curls, but I think the large would be awesome for the roots - think of a roller size...it would be like using a round brush or rollers, but it doesn't split your root clumps or anything. The small one is quite slim, would be good for sharpie-size curls, I think. I would pick the size that suits your personal curls.
For using it to place a curl, you just put the end of your clump in the middle prongs, and twirl it with the direction of your curl

leave it in a sec, then slip it out, without disturbing the curl you've just helped. I also use mine to gently pick-comb my roots and my bangs once they are dry. So it's kinda like the Swiss-Army tool for hair lol
And you're welcome, anytime.

Always happy to help a fellow curly.
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Finer, mixed with coarser strands, low-porosity, wavy curly whirly hair...3A, 2C hair. Medium thickness, medium elasticity.
S-shaped hair with corkscrew ends. My hair is even more confused than me ;p
My hair officially hates protein

Once a month is enough!
Currently a fan of TiGi products, (rock on, TiGi Curlesque styling creme!) but that will soon change - I am a self-avowed product junkie! Yay!

BIG fan of supersoaker method - thanks Rudeechick!