Unity and units:
First and foremost the unit thing is and always will be a problem. I can't give any advice about how to do a "proper" scaling, besides trying out what fits best.
It might help if you switch to actual units instead of Blender's default setting (no units). In the properties window switch to "scene view" and there chose "metric". You can also chose a different scale here.
Exporting to obj:
Exporting to obj with default settings will always export the whole scene. You'll have to select the model in object mode, then change the settings in the export window at least to "selection only". If you have active modifiers that you want to have in the final model you should check "Apply Modifiers", too. I don't know much about unity's import function but I never had a single case where I could reuse Blender's exported obj materials. I always keep that one unchecked.