What is a normal size for a retroperitoneal lymph node? i had 2 ct scan and have a stable single 13mm one and am freaking out. i hear anything above 1cm is bad and abnormal?

Lymph node "normal range" sizes are estimations. There's no actual "exact" cutoff number where a lymph node is normal, vs. one that is enlarged due to cancer or infection or other abnormality. Think of it like this, just as people can be different sizes and still considered "normal", so can lymph nodes. As long as it hasn't changed dramatically over time, I wouldn't be concerned.

Understandable as incidental findings may create significant anxiety if they remain unexplained. Unless able to interpret such findings in your clinical context or relating them to a condition that will unify your symptoms with the finding in a diagnosis would not move focus to the imaging finding, but rather try to explain your symptoms that triggered the test.