Here is a chart of the number of words in use after each garbage collection (for MIT/GNU Scheme compiling itself with a heap of 1935360 words):
This is the smallest heap that is big enough for the compilation to complete. The limiting factor seems to be the big spike on the left at about 0.9×109. We obviously wouldn't want to run with the minimum size heap; 3935 garbage collections are needed and almost a third of the total time is spent in GC. But the more frequently you collect, the better resolution you get.