on the confirm page, find the following hidden elements:
<input type="hidden" name="on0_1" id="on0_1" value="" />
<input type="hidden" name="os0_1" id="os0_1" value="" />
These are used for passing option information to payapl.
set the first one to the name of the option, "Size":
<input type="hidden" name="on0_1" id="on0_1" value="Size" />
for the os0_1 hiden element, bind it to the Size column of the eCart object.