whats a good size for a standard terrain? Details, grass, trees disappear when zoomed out editing.

I'm trying to make a smallish terrain. But the thing is when I zoom out to get a good view of it, all the details (trees/grass/terrain details) disappear and I can no longer see the grid clearly. Does that mean its too big? If so what is a good size to start off with?

No, that doesn't mean it's too big. The terrain engine is flexible, and can be used at many different sizes. The default size of 2000x2000 is fine, and if you are using the convention of 1 unit = 1 meter, this should be plenty for many situations.

The "disappearing details" is an intentional effect - the terrain engine has a system whereby details that are further away from the camera are not drawn - this is a part of the terrain system's optimisation so that it runs fast.

If you want to be able to see your grass & details from further away while editing, you can turn up the "Detail Distance" setting in the terrain's "Settings" menu (the cog button, when inspecting the terrain).

If you do this, you should remember to turn it back down to a sensible level before publishing/running your game, so that your game is not needlessly drawing massive numbers of details far into the distance - this is usually undesirable because it will degrade performance.