The fillet radius is the radius of the arc that connects filleted objects. By default, the fillet radius is 0.500 or the last radius set. Changing the radius affects subsequent fillets but not existing ones.

To set the fillet radius

From the Modify menu, choose Fillet.
Enter r (Radius).
Specify the fillet radius.
Press ENTER to reenter the FILLET command.
Select […]
Source: CAD Blog.Net – Setting the Fillet Radius in AutoCAD
Go to Source: CAD Blog.Net