There is a "drag" button if you hold shift, so it's possible to drag boxes out of corners :) Will think about how to make this more clear though!
Having a limited jump height is a deliberate design decision, to encourage combining the boxes into stair-like compositions. Likewise with the stationary sizing pads, where we want to create a more frantic back-and-forth gameplay, and where an important part of the player's skill is learning how to measure up box sizes from afar. But will take your feedback into consideration, the important thing in the end is to make a fun game!