From the man page for mount (scroll down a lot):
Mount options for tmpfs size=nbytes Override default maximum size of the filesystem. The size is given in bytes, and rounded up to entire pages. The default is half of the memory. The size parameter also accepts a suffix % to limit this tmpfs instance to that percentage of your physical RAM: the default, when neither size nor nr_blocks is specified, is size=50% nr_blocks= The same as size, but in blocks of PAGE_CACHE_SIZE