How big and how dangerous is a groin hernia and it's really painful at time?

Hernias may hurt : Hernias are not normally "dangerous." they can be if they are small and the contents get "stuck" outside the abdominal wall - that is called "incarceration." most hernias that simply go in and out - typically worse when standing and better if you lie down - are not incarcerated and do not represent an emergency. I had one for 42 years before having it repaired, & there are many types of repair.

See a surgeon: Groin hernias can range from not having any symptoms to being life threatening. Hernias are a defect in your abdominal wall. Fat or intestines can become stuck and cause pain. Once you have pain, you should see a surgeon to have it fixed!