The biggest one that you can afford. Really. See this: http://www.tomshardware.com/reviews/sata-6gbps-performance-sata-3gbps,3110-8.html
If you can afford it, the SSD should be big enough for your OS and your programs. You can redirect My Everything to an HDD for storage.
If you run apps such as Photoshop that do massive amounts of IO and caching, you might benefit from having enough space on the SSD for your scratch files or your entire project, but now we are talking big money.