What's the minimum partition size for osx mavericks when dual booting with windows 8?
OS X Mavericks requires at least 8 GB of free space to install to. At least 15% of free space is recommended for updates and other such things.

Therefore, the minimum space for OS X is 9.2 GB, or to be safe, 10 GB.



So yes, using Disk Utility to partition before you installed Boot Camp would have meant that you could have made the OS X partition smaller.

You can repartition now with software such as iPartition, which specifically works with Boot Camp partitions.