As a guide, AVT shirts and scrub tops suit the following:-

Shirt SizeSuitsPolo Shirt (Animal Studies Only)
Length (from highest shoulder point)
Polo Shirt (Animal Studies Only)
Chest (shirt size)
XXS664cm
25 inch
84cm
33 inch
XS866cm
26 inch
89cm
35 inch
S1069cm
27 inch
94cm
37 inch
M1272cm
28 inch
99cm
39 inch
L1474cm
29 inch
104cm
41 inch
XL1676cm
30 inch
109cm
43 inch

If your measurement falls in between two size, the next larger size is best.