San Buenaventura is a beautiful mission. I assume you are building a model.
Fortunately, San Buenaventura was one of the missions for which architectural drawings were done in 1933, during the Great Depression when unemployed architects and photographers were hired by the government to document historic buildings. There are drawings that show the front, back, side and interior outlines of the mission.
The scale on these drawings will give you the exact measurements which are always better than rough measurements.
You can find these drawings and additional information on the San Buenaventura mission (and other missions) under the individual mission section of our site.