There is a per file limit of 15 GB. There is a 25 TB limit per site collection. Site collections have traditionally been composed of large groupings like Central Admin or FAS but we have many more single use site collections now, which are issued on request. Separate site collections are also issued for HRCI data collaboration.
Each site collection can have up to 2,000 subsites and they would all share the site collection quota, so there is no per subsite limit.