I have just been told at my 20 week anomaly scan that there is a pericardial effusion measuring max 2.4mm. Is 2.4mm high?

Yes. Any pericardial effusion greater than 2 mm is considered abnormal. The fetus should have a high level evaluation of the heart to check for any evidence of a congenital abnormality of the heart. A pericardial effusion is usually a reflection of heart failure and requires further evaluation to attempt to identify a cause.
Not normal. A pericardial effusion is not commonly seen at all on 20 week ultrasounds. You should see a high risk doctor.