Travel was for my wife + 2 daughters, all with Premium Plus reservations (incl. 3 checked bags + 3 large cabin bags). I purchased WizzAir's Premium Plus package for everyone so that they could check their bags and stow the girl's 2 violins in the overhead bins.
Upon arrival at BUD, the check-in agent claimed that my daughters violins were 3 cm longer than allowed for large cabin bags. These were child-sized violins, so think small cases. My wife was then charged $352.75 USD to purchase 2 additional full-price seats, one for each violin.
My wife was clearly targeted as a foreigner. Several Hungarians checking at the same time had bags that also exceeded, by similarly small amounts, the size-limit boxes in question. This seems to be WizzAir's business strategy – low fares (or not so low, with the Premium Plus added), but they fish for victims at check-in who clearly can't or won't turn around and go home, and then WHAM, the sky is the limit.
In the end, the two violins fit in the overhead bins with plenty of room to spare, and the $352 in seats were never used. We have now made 2 formal complaints, but zero response. Travel on WizzAir with extreme caution.