From my original post:
Of course it wouldn't really work to keep having it scale when there are 2 or more preview lines.
I said this about the image specifically because the contact pics would keep growing. So it'd still be centered, but the same size as it was with one line. Centering keeps it symmetrical, at least which was the main issue I had with the picture. And I think the size it'd scale to at one preview is ideal.
As for keeping the messages all the same size, as you said, after 1 line also becomes unwieldy, since not all mail would fill them. Personally, I use 1 preview line and no filename for attachments, so I never have to worry about a 4th line, but I do get stuck with only 2 lines when there is no preview. I suppose those of who prefer a uniform message list would either have no filename, or have the filename count as the preview line. So either way, it's fixed at 3 lines. If there's no preview to display, a blank line (like in my photoshopped pic above) or "no preview available" would work (kinda like when there's no subject).
At 2 to 4 lines of previews, I think the person is more after having as much info displayed as possible than any sense of symmetrical aesthetic.
So I guess it's 2 issues.
1) The option for centering the contact chip and scaling it (with a set max so it doesn't go crazy) when previews are indented.
2) The option for locking message thickness at a set number of lines so the message list remains uniform.