Hi, thank you for playing!
1. The units' size matches the size of the game's art in general. Their collision box was increased for better clickability, it will be fixed on the next update.
2. Workers are there to offer a small help on gathering Gold and Gems. They do not fight. They will run away if an enemy is near. You need to take the battle far away from them in order to properly gather the resources. To put an worker and it simply takes all the resources without any kind of risk is preferable to not have it at all. I think the workers are doing their job for now.
3. I don't like automatization in games like mine, I like to click and buy units whenever I feel I need to. Also, it is a way to save money for the next stage.