"The largest monstrance in the world is located at St.Stanislaus Kostka parish in Illinois (Chicago area). I believe, at present, it is located in the parish church, which has perpetual adoration, and is awaiting completion of the Sanctuary of The Divine Mercy, which appears to be a desire of Our Lady, directly made known to the pastor of this church.

The vision for the sanctuary and monstrance began nine years ago when St. Stanislaus Kostka pastor Fr. Anthony Bus, CR, heard a call from Mary asking him to build the sanctuary. (He wrote a book about his experience entitled A Mother’s Plea: Lifting the Veil in Sanctuary (Marian Press).) The book, originally published in 2005, has been a popular title in Catholic book circles partly because of its apocalyptic theme. It has been updated with information on the painstaking effort to build the sanctuary.
The gilded monstrance was sculpted by an artist/sculptor named Stefan Niedorezo and took two years to carve from linden wood using Renaissance methods (whatever those are..maybe you know). The 'iconic' monstrance is nine feet tall and weighs 700 pounds. Malgorzata Sawczuk is the artist who applied the gilding and he also serves as project conservator. (I don't necessarily know these artists or recognize their names, but I include them in case you want to research to know more about them).
The monstrance depicts the Blessed Mother as the link between the old and new covenants. She is depicted above/over the Ark of the Covenant which, as we know, contained the stone tablets inscribed with the 10 Commandments. In the monstrance, Mary is “clothed with the sun, with the moon under her feet, and on her head a crown of twelve stars,” (Book of Revelation (Rev 11:19 and 12:1-2).
The unveiling of this monstrance took place last year, 2008, on the feast of the Visitation. The ceremony was televised live on EWTN and the Latin American TV station El Sembrador. Relevant Radio also provided U.S. coverage. Mass was celebrated by Cardinal Francis George.
As I mentioned, perpetual adoration before Jesus in this monstrance, is to be held in the parish church (except when Mass is being offered) until the Sanctuary of The Divine Mercy, requested by our Blessed Mother, is completed. As of this date, I have no idea how far along they are or if the Sanctuary is finished or ??? I am definitely going to go, one of these days, to adore Jesus in this most beautiful place and a place specifically requested by His Mother! I think it is so telling, for our times, that Mary requested the specific name She did for the Sanctuary - The Divine Mercy !!! (and, how often do you get to adore Jesus in a host that's bigger than a dinner plate???!!!)

Notice, when you look at the beautiful figure of Our Lady, that Jesus is positioned to be both in Her heart and in Her womb.
Below are some links that will show you beautiful pictures as well as some of the information. I've tried to label the links somewhat, so you know what they are, but you'll find out soon enough by clicking on them.
ENJOY !!! (I dare you not to get spiritual goosebumps!!) grin.."
http://www.amothersplea.org/ (home page of the Sanctuary of The Divine Mercy)
http://www.catholicnewworld.com/gallery.aspx?gallery=7 (various pictures of the Marian Monstrance in progress, etc)
http://www.amillionsouls.com/ (another homepage of the Sanctuary of The Divine Mercy, w/link to sign up for adoration and other links)