Adding additional outlet - max wires and box size?
Read inside the box.  If you install 14 AWG conductors to the new box, instead of 12 AWG conductors. You'd need at least a 22 cu.in. Box, or a 3" x 2" x 3 1/2" device box.


8 for current carrying conductors.
1 for grounding conductors.
2 for device fill.


11 total, times 2.00 cu.in. for 14 AWG conductors = 22 cu.in.

However, since you're using 12 AWG, the calculation looks like this...


6 for 14 AWG conductors x 2.00 cu.in. (12 cu.in).
2 for 12 AWG conductors x 2.25 cu.in. (4.5 cu.in).
1 for grounding conductors x 2.25 cu. in. (2.25 cu.in.).
2 for device fill x 2.25 cu.in. (4.5 cu.in.).


Total = 23.25 cu.in.

For more information on box fill, check out this answer

As for connecting the new wire. There should be no problem attaching it to the receptacle. However, since you're using 12 AWG conductors, you'll have to use the screw terminals. 

If you want to do pigtails, you can. I know some guys don't like the backstab connections, so if you want to avoid using them you can do pigtails. Ideal 76B® Red Wire-nut® Wire Connectors are rated for 2 to 5 #14 conductors, but check with the manufacturer of the connectors you're using.  Again, if you're going to do this, you'll want to use #14 conductors instead of the #12 you've installed.