Depending how much you want to install along with the OS, but I wouldn't make it less then 40gb if you have the space 20 gb would be the minumum.
Once you have finished installing everything you want, you can use a disk image program to image the drive and restore on another.
I like Acronis True Image Home 2009, it works with W7, but it's not free.
It will restore a larger drive to a smaller one as long as the used space is less then the capacity of the new drive.
NOTE:
I have no knowledge as to compatability with SSD.
You could use the Windows Easy Transfer wizard to backup your files and settings and clean install W7 on the new drive and transfer the backup back to the new drive. It actually works very well and reinstalling W7 will only take 20 minutes or less.