Make stimuli sizes / locations more user friendly?

[suhasshrinivasan]

Wondering if we can make stimuli parameters such sizes and locations more user friendly. With the current implementation, I get to understand the values only by trial and error by looking at the output.

Idea 1: Make it explicitly relative to canvas_size, this way changing canvas_size retains the size/location semantics and the latter don't have to be readjusted. It's also more intuitive this way.

Idea 2: Also accept categorical values such as "center", "top-right", etc for location param and similarly "large", "medium", "small", etc for "size" param. Of course, integer based parameterization should also be accepted, and strings only map on to them in case they are provided.