I usually base it around the board width of the themes I've got installed. If the board width is on the bigger side, I go with bigger rank images. For example, on Poog, I'm using a theme with a smaller board width, so I made the rank images pretty small. (They're 100x22, almost the size of a affiliate button) Here, I'm going to use smaller rank images once they're done because the board width is smaller due to the columns I have enabled on the forum. Going by the theme on your forum, I'd go with rank images that are smaller so the rank images aren't too big and take up most of the user information. I hope this helps.