The Size column is the last column in the message list (at least in my claws layout). The size value it shows for each message is not truncated to fit into the column width, but bleeds out to the left, overwriting the date if it is close enough. (At least that is what I'm seeing on my Fedora 20 box with claws-mail-3.10.1-1.fc20.x86_64 installed).
I have the same problem but with the message number bleeding to the left. It's real annoying.
*** Bug 3506 has been marked as a duplicate of this bug. ***