In case you're creating one partition, should it be as big as possible, or should a certain percentage of space be left unpartitioned?
I recommend the former (as big as possible) to allow the drive controller to more efficiently utilize free blocks during file updates. Anand from AnandTech wrote an excellent article about that last year. (Note that the link goes to a part of the article where this is explained, but I recommend to read the whole article for gaining better understanding of the concept.)
If you are worried about leaving some space for the drive controller to handle wear leveling, then you should not need to worry about that since most SSDs have space reserved for that already (i.e. 30GB drives can be 32GB large in reality).
As a general rule you want to format your SSD to 80% of the advertised size (the 20% rule stated in reverse) for performance reasons.
http://www.anandtech.com/show/2899/5 discusses capacities and overprovisioning in regards to lifetime/reliability
http://www.anandtech.com/storage/showdoc.aspx?i=3667&p=3 discusses free space effecting speed on Trim enabled drives:
presumably this issue is even worse if you don't use Trim.
If you already use the SSD at full capacity you may have to do a secure erase to allow the firmware to treat the "unused" space as unused.
Warning following the instructions you might find after googling secure erase or sanitary erase might lead you to the point of having an unusable SSD. you are much better off making sure a new drive is never formatted to the full capacity so you can avoid this procedure. Given that disclaimer some quick links to read in addition to the Anand articles would be:
Nothing short of resetting the SSD to this "factory fresh" mode turns used space back into unused space. You can't just reformat the disk or resize the partition or use any traditional method you would have used on rotating disks in the past.
As big as possible, unless you have a specific reason to leave a certain percentage unpartitioned.