Have you talked to any other current PhD students at your school?
For my master thesis proposal in chemistry, it was around 20 pages (single-spaced, 12 font, figures, tables, 10-20 references from journal articles or textbooks), however, I know couple of people only had around 5-10 pages for their proposal and they were able to get away with it.
a typical Master thesis consists of 3 chapters (minimum), 70-100 pages, double-spaced, 12 font, figures, tables, and w/ 30+ reference
a typical PhD dissertation consists of 5 chapters (minimum), 150+ pages, double-spaced, 12 font, figures, tables, and w/ 50+ reference