How dangerous is an enlarged left atrium?

The size is . Not the problem, the issue is what caused it to become large. Significant mitral regurgitation is a possibility as is hypertension. An enlarged left atrium increases the risk for atrial fibrillation. See your doctor .
Clot. And if there is narrowing of the mitral valve (mitral stenosis) then a larger left atrium increases the risk for clot formation and stroke.