Right now in Marvin, it's difficult to estimate any book's relative length when compared to other books. You basically just stumble into a book but have no clear idea how long it's going to be. Even in the expanded mode of Marvin's Library view, the little dots meant to indicate your reading progress have the same combined length of 100%, from screen edge to screen edge, for every book -- no matter if it's a 100-page or a 500-page book. The mini progress-bar announced for Marvin, version 1.2, will help alleviate this problem, but not quite.
On this issue, too, I would recommend Kris to try and emulate the Kindle app. Amazon came up with the ingenious "locations" for Kindle books (later even "real paper page numbers" for some titles), which is an exact way of indicating every book's length, but this is not applicable to EPUBs and Marvin.
However, Marvin might employ the Kindle app's way of indicating book length in Library view: if a book has 500 pages, it might have 25 dots to indicate 100%. However, if a book only has 100 pages, then only 5 dots indicate 100%. That way you can see at a glance approximately how long each book is. Reading progress through the books is indicated in the Kindle app by white dots gradually appearing in a different colour (blue). Blue dots = read; white dots = unread; a full row of blue dots = you've finished reading this book; a full row of white dots = you haven't started reading this book yet.
Thank you for considering this, Kris.