I'm running PS CS6 on a 13" Retina Macbook Pro with a 128 GB internal SSD. I've pretty much filled it up, and am buying two external hard drives to migrate my old files off (with a backup). Having freed up some hard drive space, I'm thinking I might partition off some hard drive space for a scratch disk. I've read that it's better to use an internal SSD than an external drive (USB 3.0), so my question is-- how much space should I give it? I've seen recommendations all across the board on the Internet from about 2 GB to several hundred GB. Realistically, I do need space on my hard drive for operating system and stuff, so I can't give it the whole thing. I'm also considering partitioning off some of my new external hard drive.
Having scratch point to an SSD is a great idea for keeping Photoshop responsive, but you really don't have enough room to do it. Photoshop eats up scratch space like popcorn.
Also, I'd STRONGLY suggest against doing a reconfiguration that will potentially run your system disk low on free space. The OS HATES to run out of space.
I just saw that a quite decent 512 GB SSD (Crucial MX100) is now available for roughly US $200. That blew me away. It might be a good time to think about upgrading, assuming that's possible with a MacBook Pro.