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It's really going to depend on the gate staff on the day. The people checking you in don't work directly for Wizzair, but for a third party handling company - meaning they don't really give that much of a shit about customer service, which can swing both ways - they'll either not care enough to even check the size of your bag or will heartlessly kick you off the flight if you don't pay the excess fees. In my experience a bag that is just a 'little bit' too big will probably not be noticed, but that's a big probably, particularly if you get away with it on the way out but not on the way back.
I don't have experience with Wizz directly but I'd be surprised if they didn't offer free gate bags (i.e if your bag is too big for the cabin they chuck it in the hold for nowt - inconvenient for sure, but free) but I suppose you'd have come across that information if they did offer it already.
Basically what I'm saying is it's a gamble based on the judgement of some underpaid check-in/gate girl and while you might get away with it - I'm willing to say you probably will get away with it, but you definitely need to be prepared for the eventuality that you might not. It might be worth simply getting or borrowing a backpack that is within the requirements - you'd not be losing too much space. Either that or carry £60 in your pocket. If they do try to charge you, it's worth claiming you can't afford to pay it - again, nobody you'll be talking to will work for Wizz directly so they don't really give a fuck if you pay or not, they want you on the plane so the plane can leave. But there's jobsworths everywhere, so if that doesn't work you'll have to miraculously find the money if you want to fly.
Just remember, this sort of thing is the reason low cost airlines are low cost. They justify their prices by flying as light as possible to save fuel, and charge you extra if you use more. The cabin bag size will likely be an arbitrary number, but the cut off has to be specific. You can take whatever bag you like on a BA, but you pay an extra couple of hundred quid to do it.