It’s usually the height of a soft sided suitcase that causes issues - sometimes people over-pack or expand the suitcase and whilst it will fit in an Airbus locker it won’t fit in the E-jet locker. More proactive crew will offer you a bin bag to decant stuff into if your suitcase is too fat for the locker but be warned these bags are clear so everything will be on show!
The space under the pairs of seats is totally uninterrupted so sometimes this will be a better place for larger bags especially if the seat next to you is empty.
The vast majority of wheelie suitcases will need to go in the locker sideways. Short and stubby cases usually end up fouling the locker door and will have to be put under seats or in the hold. Also note the very last two lockers are slightly shallower than the rest (Yo-yo pushchairs and similar will not fit here).
E170s have a wardrobe which can sometimes be a solution to troublesome items but no such facility on the E190.