Looks like ER8 is the smallest:
New Straight Milling Shank C10 ER8 100L Collet Chuck | eBay

The thing to do is have a go at making your own ER8 or ER11 collet chuck.

Quote Originally Posted by njhussey View Post
going down the DIY laser route today. Could make a real lightweight router then ;)
I'd love to see a router made with a laser...