There are 16.39 cc's in a cubic inch; one cc is 0.061 cubic inch (hence the .061 engines -- they're 1cc).
6.5cc is .397 cubic inch, or a 40; 6.6cc is 0.403 cubic inch -- or a 40, depending on who's selling.
(1 inch = 2.54 centimeters, by definition. (2.54 cm)3 = 16.39 cc, or 16.387064 for the picky folks. 1/16.39 = .061).