Hello!
Thank you for your feedback (and for registering here!).
Of course it would be best if they would only water crops that needed watering
This is already done in some form: the sprinkler scans around to check if there are plants that are not spoiled and have their water level below 100%. If there are no such plants, watering will be skipped (no water/electricity consumed). Even when you manually press the button it will not spend water/electricity if there are no valid plants nearby.
However, it doesn't check for plants that have harvest. We will check and improve if necessary.
Right now I have to leave them powered off, then power them on, click 'sprinkle now', then power them off again.
I don't think having a button to water only once is a good idea. The presence of such a button will create confusion and make players approach it wrong. Our intention is to provide convenience and reduce micromanagement.
Please note that in the latest patch (released just a day ago, before you posted this topic)
we've made that sprinkler consumers 3 times less water. It will last (when filled with water completely full) for 5+ days even when it will be watering every hour plants around it.
Regarding their size causing problems for you when walking around—if you can, please share a link on a screenshot. We will check and consider adjusting the collision bounds.
Regards!