When making a game similar to Hearthstone or Magic the Gathering how large should the board be and should it be a plane or a terrain?
The default scale is 1 Unity unit = 1 meter. So if you’re not using a different scale, just think about how many meters your board should be.
As far as plane vs terrain, the answer is usually neither. The typical workflow is create the board in an external 3D modeling program, export it as FBX, import it to Unity, and use that.
I guess the board would be quite small based on the Unity unit. Does it make sense to have the board around 0.5 Unity units and a card would be even smaller?