If 'Frozen' was live action movie instead of animated, who would you cast to play Elsa and Anna? Their voice actors or someone different?

Their voice actresses. Kristen Bell and Idina Menzel actually don't look so different that it wouldn't be a stretch to call them sisters. Kristen has that youthful, perky, sunny look that would make it possible to play Anna and Idina has that "serious woman" persona down pat, which we know from Enchanted.

I could see Kristen Bell{though she voiced Anna} playing Elsa......
Lily Collins, Jennifer Lawrence = Anna

Emilia Clarke as Elsa and Dakota Blue Richards as Anna.

Emilia Clarke plays Daenerys Targaryen and Dakota Blue Richards was in the Golden Compass.

I'd keep the voice actors. "Let it Go" is truely a hard song to sing due to the two high note, plus trying to stay out of your head voice for the whole thing. Plus, they already announced that Idina will be playing Elsa in the Broadway show Frozen.

Someone else who didn't play them already

For Elsa I would either cast, Dove Cameron or Kristen Bell. For Anna I would either cast, Karen Gillian or Bonnie Wright.