Though not made for a holy communion, I thought it best fit in this category.
I made this some 15-20 years ago so I hope I can remember the sizes.
The bottom tier consists of six 12x18x2 sheets, 3 on each of two boards. On each end is a large book pan cake. Next up is a 36x18x2; then a 24x18x2; a 12x18x2 topped w/a 12x12x2 turned sideways;.
All are iced in buttercream; piping is also b'cream. Flowers are air-dried b'cream mums and roses, along w/some gumpaste lilies and waferpaper fall leaves. The books have frosting sheet pix - one dedicated to each family that was leaving. Topper was a metal filigree church. If the sizes are right - this served about 700! The book cakes were sent home w/each family.