I'm not sure there is a "typical" commentary by which to judge some standard length. I have an old copy of a Haggadah published by the Staten Island Yeshivah from the 1940s or so (I have to remember to pick it up and look at the actual date), which is pamphlet-length in its entirety, despite having a good deal of commentary throughout, and the "Torat Hyim" Haggadah, which is over 200 pages long, despite its mostly short commentaries included. Of course, in the latter, each passage of the Haggadah has several commentators' writings.
My personal feeling is that a Haggadah's commentary ought to be engaging to the reader, however long it may be. To the point, the Nechama Leibowitz Haggadah, which my wife favors, is 159 total pages and contains, "over 100 analytical questions relating to the Passover Seder and the Exodus from Egypt...extracted from Nechama's renowned weekly Torah study sheets." Yet its style keeps the reader engaged, and it can be used to study from in preparation for the Seder or to pull up a quick Devar Torah at the Seder itself.