Hi,
5/32 inch is the smallest floor spacer you can use.
I also recommend you use a mid gray colour grout. It doesn’t stand out and is the easiest colour to keep clean.
When you buy the grout, tell them what size tile spacer you are using. There are a few types of grouts out there for different size spacers, so it’s important you get the correct grout for your job.
http://www.tiletools.us/index.cfm?Action=ViewCategory&Category=18
Regards