"I am a plus size and was wondering if it could be seen in a ct scan of the abdomen or if I need a scan of my back"
Hi, an ultrasound depends on seeing the difference between the lump and the surrounding tissue. If a lipoma is in a layer of subcutaneous fat, then it's harder to see - especially as incidental. A CT might very well be similar, since a CT sees density and the density will be similar.
Also, as backhurtz mentions, a CT isn't full body so it depends if the CT slices were at the same spot as the lump.
If you had abdominal scans and the lump is on your back, it may not have shown up or been seen during the exam.