So I have a MacBook 13" that I dual-boot with Windows 7 and Mountain Lion. I'm in the process of replacing a majority of the inerts. I've already put in 16GB of RAM (I work at an ISV and need as much as I can get somedays). My next project is replacing the optical bay with an SSD.
My only trouble is, I don't know how big of a SSD I need. I plan to have both OS's on it along with all of my most used programs which include:
Windows 7 (8 eventually, well maybe) side:
I.E. (grosssssssss, but I need it for development testing)
iTunes or Zune Software (depending on if my Zune HD dies in the near future)
SOTI Pocket Controller
If you really want to calculate you can just Google name of program and add 'requirement' to see how much space the program needs and add it all up but keep in mind you need to add an extra space in the end just in case.
As nyxana alreay mentioned just take a look at the system reqirements for each application and utility that you plan on installing. Then add all the storage requirements to determine minimum capacity you will need for installation. That is just the start. Allow for expansion and upgrades.
The general rule of thumb is purchase the largest capacity ssd you can afford at the lowest possible price.