As others have said, LCC have different dimensions requirement for free cabin bags (aka personal items bag, “meant” to exclusively go under the front seat) but as far as I know, IATA cabin luggage size dimensions 56x45x25) apply for large (paid for) cabin bags (edit: Apparently Ryanair large cabin bag needs to be slightly smaller, 55x40x20, albeit I have brought my usual large IATA Cabin bag in the very few flight with them since COVID).

You can find compliant Under Seat bags for the two biggest/most common LCC in Europe on Amazon, specifically highlighted as EasyJet or Ryanair complaint.

for reference, as of today, the dimensions are:
EasyJet 45x36x20cm

Ryanair 40x20x25cm

for comparison, BA Personal Items Bag, meant to go under the front seat, has the following dimension:

British Airways 40x30x15cm