What is a good amount of memory(gigabytes) for a SSD?I already have a 1 terabyte hard drive.

Solution
Grab a ~256GB SSD, they are reasonably cheap these days, you really shouldn't go less than 120GB if you are running it as the main disk in a windows machine.
It really depends upon what you are going to install to it. Normal Operating System plus a few programs - 120GB is fine. If you have a lot of programs to add to it, you may need 250GB.

I have Windows 8.1, Microsoft Office, Microsoft Visual Studio Professional, Adobe CS6 Master Collection + a few other programs, and use about 85GB of 120GB. You can utilize your 1TB HDD for the other programs....
 
If you're talking about SSD capacity, then I'd say these days 256GB would be a good entry level point IF you are into AAA gaming. Many new AAA titles have a ~50GB install size now, so a 128GB SSD (with ~120 GB usable space) will be just about 75% full with just a single game (given that Windows 8.1 and a few apps will likely chew ~30GB + another 10GB for the swapfile and user folder etc).
If, on the other hand, you are just building a simple MCPC (and will solely be used for that purpose only), then 64GB would actually be just fine.