Source for ECB? FRED says they've got about 6.458 trillion euros [0] and the english version of the ECB website says they had 4.6 trillion at the end of 2019 which also matches the FRED and they haven't updated it since then [1].

[0]: https://fred.stlouisfed.org/series/ECBASSETSW

[1]: https://www.ecb.europa.eu/pub/annual/balance/html/index.en.h...