Hi David,

It is recommended that you keep 25% of your drive free. Also, 10% over-provisioning is also suggested. So plan on 65% of the total disk space available for Windows, programs, and data.
Bigger SSDs are usually faster than smaller ones, so that is another consideration.

Take a look at how much space you are using now and make your decision based on that number. If your budget allows for a bigger drive, then I would go that route. It's really easy to fill those suckers up these days and a little wiggle room never hurts.

I hope this helps,
Richard