Depending on how big of a curl you're looking for, depends on the size rollers to get. The larger the roller, the looser the curl. I have basic ones by Conair --- they're like 1 1/2" . I just basically use a round ceramic barrelled brush to flip and curl my ends --- it's a little easier since my hair doesn't hold rollers well.
I also have the 1 1/2 rollers from conair as well and my hair has layers in them as well. Using them, defines my layers. I would recommend them. You could also use an 1 1/2 curling iron as well and still get the same effect.