They have a measuring sizer in which you are expected to place your item if asked. If it doesn't fit, you have to gate check the item at an additional charge. They don't ask as many people as previously, although it seems those who don't pay for any luggage option would be a good target.

I once measured the old sizer (55x40x20 only) and found it was 1cm larger in two of the dimensions and 3cm larger in the other. They've now replaced these with a new one that measures both 55x40x20 and 40x25x20. I haven't been able to measure this yet.