What is the "short-axis" diameter of a lymph node that is 2.7 x 1.9 x 0.6 cm? is this large for a mesenteric lymph node?

"short axis" : diameter is just what it sounds like... the shortest measurement of length, width, or height, which in your case is 0.6cm. Many find the short axis measurement a more reliable indicator of abnormality than the long axis measurement. Generally about 5mm is the upper limit of normal, so a 0.6cm node would be mildly enlarged. There are many potential causes, most benign. Shape is also important.