This is really very dependent on where you want your mountain and what else you are placing in the scene. What I've done is enlarge it to the sort of size I would start to build my scene around. I'd still be adjusting and moving throughout the scenebuilding process so this is just to give you an idea what kind of sizes to consider.

In case the dimensions aren't visible in the image the size is

X = 404.84	Y = 151.39	Z = 404.84