A problem for anyone trying to answer this question is that most of the staff who you see dressed in Wizz Air uniforms aren't actually employed by them. They're working for their handling agent, Swissport.
You could find that the Swissport staff at Luton interpret Wizz Air's rules very differently from the way that their colleagues at Malpensa do, meaning that you might get away with taking your bag on board in one direction but not in the other.
Similarly, there might be someone here who's been lucky with the Wizz/Swissport staff at Southend whereas those at Luton could be incredibly officious.
Ryanair used to use Swissport as their handling agent. (They switched to using OmniServ last year). Ryanair/Swissport staff at some airports were extremely flexible and would probably have turned a blind eye to someone taking an elephant on board, whereas their counterparts at other airports measured every item of luggage down to the last millimetre and clobbered passengers for excess baggage charges at every opportunity.
So, irrespective of any experiences that ABers might be able to report on this thread, it might be unwise to assume that there's a common policy at all airports used by Wizz Air.