Is a 3.8cm kidney tumor considered big? What are the chances it's malignant and spread? No symptoms urine was fine. Mom had MRI this morning to tell.

Depends. Size of a kidney lesion is of note, but usually not the key factor in determining whether it is benign vs malignant. Other factors such as if it contains fat or if it is purely fluid vs has soft tissue components are more important. The MRI should answer these questions for you.