"stand up from prone" is an MEA. I don't know about you, but it takes me a lot less time to straighten my legs than it does to climb up off the floor.
Now, 'crouching' hasn't been defined in D&D, so it's not going to be explicitly stated in the rules - it's going to be a DM's call. I'd suggest crouching down behind a low wall and timing how long it takes you to get to a standing position. If it's less than a second or so, I'd call it the equivalent of a 5' step - meaning you can combine it with a full action just fine.