2x6 or 2x8 Sill Plate on 8" foundation wall?
A 2x8 plate is not needed. In calculating the strength of the plate/anchor we look to "develop full strength" for resistance. The key factors are the thickness of the plate, size of anchor, size of washer, depth of anchor AND edge distance from bolt to edge of wood plate. 

Edge distance required to "develop full strength " is 4 diameters. So, using a 1/2" anchor, you only need 2" of edge distance. That's why a 2x12 plate isn't even better. 

(However, make sure the anchor bolt is not pushed up tight against the edge of the concrete wall or up against the face shell of a concrete block. You need a minimum of 1  1/2" of concrete to flow around bolt to develop full strength in concrete wall and 3/4" of grout in concrete block wall.)