In the comment boxes on a recent discussion on a popular Catholic blog, there was some reaction to a recently built church that included a rather large baptismal font. One of the folks responded that she believed the purpose of the large baptismal font was to act as a "giant holy water stoup", since it was also placed near the main entrance of the church. If you want to get technical, the font can be thought of as a "stoup", but the font itself has a very important meaning, being the location where baptism is actually celebrated. I recall the discussion from Built of Living Stones, emphasis mine:
The baptismal font and its location reflect the Christian's journey through the waters of baptism to the altar... Through the waters of baptism the faithful enter the life of Christ. For this reason the font should be visible and accessible to all who enter the church building...Preferences aside, large fonts like the one discussed probably enable easier baptism of adults by immersion or pouring, and also serve as prominent symbols, reminders of baptism for the faithful (of tomb and womb), which is why they are also typically (but not always) placed near the primary entrance of a church and oriented in the direction of the altar. The little stoups next to doors came much later!
Water is the key symbol of baptism and the focal point of the font. In this water believers die to sin and are reborn to new life in Christ. In designing the font and the iconography in the baptismal area, the parish will want to consider the traditional symbolism that has been the inspiration for the font's design throughout history. The font is a symbol of both tomb and womb; its power is the power of the triumphant cross; and baptism sets the Christian on the path to the life that will never end, the "eighth day" of eternity where Christ's reign of peace and justice is celebrated...
The font should be large enough to supply ample water for the baptism of both adults and infants. Since baptism in Catholic churches may take place by immersion in the water, or by infusion (pouring), fonts that permit all forms of baptismal practice are encouraged.