A5 = small
B5 +medium
C5 = large
D5 = XL
Somewhere on the order the above codes will appear after the item number after a 3 letter code in upper case which I think refers to the colour eg item 123456 XYZ A5 would refer to a size small. Also the same format 123456 XYZ 16 refers to a size 16. Hope that helps but I agree they should show the size and colour info in the order details.
this has been happing for a while & no-one seems to know whats going on but Q are aware of it