G'afternoon Adimus,

the in-game resolution specified for each region won't necessarily appear in the video/resolution section within wow, but i believe wow will still render to the specified resolution (that is what i was told over a year ago)

for now, just set the in-game resolution field to the size you want, which should be about 25% of the main region dimensions.

ie:
for your region that is 1360x768 i would set the in-game resolution for each region to 680x384

i will attempt to get confirmation whether or not this does perform as expected