Getting Error message: error in an expression in Process model properties at process start form : neither variable(s) or rule(s) found: size

I had this error as well; when attempting a related action that goes right to a process start form, the same generic, unhelpful error came up.  When I checked the process model, it gave me a little more, that there was some error in 'showwhen' (can't remember exact error text).  I had to do an expression search and finally found it buried, several layers (child interfaces) down, it was a single error in my keyword syntax (which I prefer), essentially I had a line like this: 'showwhen: showwhen: true' .  Once I corrected that, everything fell into place but it would have helped a LOT if I could know the interface causing the error, and the line number :).