What is large mixed echogenic mass?

Difficult to say: Sounds like an ultrasound report maybe. It may mean that there is a mass with some solid and some cystic (fluid-filled component). It depends on the modality of the test (ultrasound or ct) and the location as to the diagnostic significance. Please discuss with the doctor who ordered the test and maybe resubmit this question and include more relevant information to get a good answer.

Many things: Ultrasound describes masses this way. Ct would describe it as mixed density. It is important to know what body part was being imaged (pelvis, abdomen, thyroid, etc) to answer further.