npctalk use the same system like when a normal player talks, if you modify the SRC to increase the value, the displayed message for players will increase too.
Anyway if you still want to do this, go to src/map/clif.c and search the following:
case AREA_CHAT_WOC:
map_foreachinarea(clif_send_sub, bl->m, bl->x-(AREA_SIZE-5), bl->y-(AREA_SIZE-5),
bl->x+(AREA_SIZE-5), bl->y+(AREA_SIZE-5), BL_PC, buf, len, bl, AREA_WOC);
break;
As you can see, you can see the value 5, thats the value that what is the range in any direction, so if you want to change the range, you only have to change that value.
Dont forget to back up your original files, and to recompile. You can test and play with the values to the choice that you like more.
PD: Anyway, you can create another case and another map_foreach... but the modifications are a little more large in the source...
(Sorry if some words are incorrect or I made no sence in some parts)