I use KMail but not any other PIM applications. My layout has the folder list down the left, then the message list above the current message. I already resist developers' attempts to waste my valuable screen space, and I'd resent losing any more of it to bells and whistles I'll never use, or to daft ideas of modernity.

"Is it really useful to see message count/size in the folder view?" I don't bother with the size, but the counts are essential to show where I have new messages.