Typically Windows 7 + typical programs take around 35 Gigs, so add to that installed games to determine size required. NOTE this is with some space saving tweaks.
one think you need to factor in is: (A) Just like HDDs real space is less than advertized. A 128 gig is 119 gigs formated. (B) must leave at least 15% of SSD unused. This is so that Garbage collection, wear leveling and Trim can work their magic. ELSE you take a chance of considerable slow down or extreme case clobbering the SSD.
The two programs that I've used are EasyUS and EZ-Gig-II (Now -IV). Ez Gig is about $20 for the Sata->USB cable. But is well worth it, I got mine free with one of my SSDs. The program used must allow for resizing the partition and allow for selecting which programs/data to migrate. A 500+ SSD runds 400+ bucks.
I STILL recommend a clean install over the Migration!
Space saving tweaks:
1) Disable hibernation. This save an amount of SSD space equal to the size of ram installed. IE if 8 gigs of ram, save upto 8 gigs SSD space.
2) Manage your virtual Memory (also called page file and swap file). Windos normally alocates 1.5 X Ram, so for 8 gigs of ram that takes 12 Gigs of space. I normally set Min and max to 1024 mbs (! gig) so saves 11 gigs.
3) I disable restore points (alternative is to limit amount of space thay can take, ie 1 gig). Windows requires 300 mb per restor points, just 10 restore points will equal 3 gigs.
Just these three tweaks can save upto about 21 gigs for a system that has 8 gigs of memory installed. If like me, I have 16 gigs ram so saves considerably more.
These are very simple to do and instructions are very easy to follow, just google ie " manage page file" or "disable hibernation" or "disable (or manage) restore points.