It really depends on how many games you have, but a 500gb SSD is a good start.
I have a 500Gb 850EVO and 1.5TB secondary HDD. Recently I had to buy another 750EVO (which is basically the same speed from my experience)
I use a tool called Steamtool to move games between SSD and HDD. http://www.stefanjones.ca/steam/
It enables playing games seamlessly on steam whichever drive the files are on without steam being aware of it. You can move large games that you play seldom to the HDD from the SSD to make more space on the SSD for new stuff.
With SSDs there is really minor real life difference (in gaming) between the EVOs and the Pros I would personally rather get a 1TB 850EVO than 500GB 850Pro.
Gabor