First, I am on Windows not Mac, but I also have a 256Gig SSD boot drive
1 - Everything I have installed (Win8.1 + Adobe + MS Office + Misc) uses 42Gig
2 - Drive C space http://forums.adobe.com/thread/1007934?tstart=0 see reply 4 (I presume similar for a Mac)
3 - USB3 and eSata are fast enough connections to use external drives http://forums.adobe.com/thread/1117813 - http://ppbm7.com/index.php/tweakers-page/84-disk-setup
Premiere puts temporary files in 3 places. I'm familiar with Windows, but I'm sure it's similar for the Mac.
1. Some temp files in the boot drive, user's folders. These aren't very big, and you can't change the locations.
2. Media Cache files, and the Media Cache Database. This can get quite big if your projects are large. These files are accessed a lot while editing. You can choose where they are saved.
3. Preview files. This also depends on the size of your project, the video format and how much rendering you do. You can also move these where you want.
Others may have different thoughts, but I would say you need about 128GB for the temp files to feel comfortable (I'm talking about HD-sized files). If the 256GB SSD is really fast (like a Samsung 950), and the projects are not 4K and not huge you could squeeze everything except the media on it. Otherwise 2 and 3 can certainly go on an external drive, but the drive and the connection must be fast.
The other issue with Media and Preview temp files is that you have to manage the locations, or else they default back to the C drive. The settings are kinda sticky from project to project, but not always. You also should clean them out between projects to save space.
I have seen a lot of comments on #2 above and in the past hard drive era many suggestions were to dedicate a hard drive to these files, but in my simple testing where I placed these files on a terrible transfer rate USB 2 connected very old 2.5" hard drive, I could not find any degradation in editing performance. Maybe there are occasions or some media types that would be sensitive to this performance-wise but I just generally leave it on the default boot SSD and clean it regularly. If anyone can provide information to the contrary, I would sure like to know about it.