First convert the paper size to centimeters. 1" = 2.54 cm
8.5 * 2.54 = 21.6 cm
11" * 2.54 = 33 cm
The building is 100*100 = 10,000 cm long
20 * 100 = 2,000 cm wide
Next we have to see whether the length or width of the building governs which scale to use. Obviously we'll put the length of the building on the long direction of the paper.
33:10000 as X:2,000
33*2,000 = 10,000X
X = 66,000/10,000 = 6.6 cm
6.6 cm is the width of the building if 33 cm is the length. So the length governs because if we try to use the width of the paper to fit the width of the building, the length of the building would be much longer than the length of the paper.
A scale where 33 cm = 100 m would put the building on the paper with the edges of the building at the 8.5" width of the paper. Now if you want some border on the paper, you could use a scale such as 30 cm = 100 m. which would give you some border between the edge of the building and the edge of the paper. In that case, the building length would be 30 cm and you would have 1.5 cm border at the top and bottom of the 11" paper.