we should diagnose & treat pericarditis based on etiology;
is it idiopathic? if yes so do serial ecg and treat with NSAIDS cause it has viral causative.
is it uremic? if yes so do ecg,echocardiography and treat with dialysis.
although we don't have complete medical history in q stem but you're right "n elham" ,seeing enlarged cardiac silhouete means that at least 250 ml of fluid has been accumulated in pericard and echo will help.