Probably 12 or 14 AWG but without actually having had any in my hands to measure I couldn't be absolutely sure.
I'd guess those stacks would be 2 1/2 inch diameter meaning you want 2 1/2mm inside diameter ferrules.
If you can get them from an actual store (rather than the interweb) then you have the luxury of being able to take a ruler with you to measure them before you buy them.