Lets say that the X coordinate of pev_mins is 100 units to the left of the entity. That means that it is -100 with respect to the entity's X coordinate. That means the pev_mins X coordinate is -100 and the pev_absmin X coordinate is ent_x_coord - 100.
Code:
//ENGINE
ent = create_entity("info_target");
entity_set_string(ent, EV_SZ_classname, "safebox_blue");
entity_set_int(ent, EV_INT_solid, SOLID_BBOX);
entity_set_int(ent, EV_INT_movetype, MOVETYPE_FLY);
origin = {1663.0, 3411.0, 679.0}
entity_set_origin(ent, origin);
mins = {-255.0, -115.0, -71.0}
maxs = {255.0, 115.0, 71.0}
entity_set_size(ent, mins, maxs); // This sets the blocking size?
mins[0] = origin[0] - 255.0;
mins[1] = origin[1] - 115.0;
mins[2] = origin[2] - 71.0;
maxs[0] = origin[0] + 255.0;
maxs[1] = origin[1] + 115.0;
maxs[2] = origin[2] + 71.0;
message_begin(MSG_BROADCAST, SVC_TEMPENTITY);
write_byte(TE_BOX);
write_coord(floatround(mins[0])); // boxmins
write_coord(floatround(mins[1]));
write_coord(floatround(mins[2]));
write_coord(floatround(maxs[0])); // boxmaxs
write_coord(floatround(maxs[1]));
write_coord(floatround(maxs[2]));
write_short(999999); // life in 0.1 s
write_byte(255); // r
write_byte(0); // g
write_byte(0); // b
message_end();
//FAKEMETA
ent = engfunc(EngFunc_CreateNamedEntity, engfunc(EngFunc_AllocString, "info_target"));
set_pev(ent, pev_classname, "safebox_blue");
set_pev(ent, pev_solid, SOLID_BBOX);
set_pev(ent, pev_movetype, MOVETYPE_FLY);
origin = {1663.0, 3411.0, 679.0};
engfunc(EngFunc_SetOrigin, ent, origin);
mins[0] = origin[0] - 255.0;
mins[1] = origin[1] - 115.0;
mins[2] = origin[2] - 71.0;
maxs[0] = origin[0] + 255.0;
maxs[1] = origin[1] + 115.0;
maxs[2] = origin[2] + 71.0;
set_pev(ent, pev_absmin, mins);
set_pev(ent, pev_absmax, maxs);
message_begin(MSG_BROADCAST, SVC_TEMPENTITY);
write_byte(TE_BOX);
engfunc(EngFunc_WriteCoord, mins[0]);
engfunc(EngFunc_WriteCoord, mins[1]);
engfunc(EngFunc_WriteCoord, mins[2]);
engfunc(EngFunc_WriteCoord, maxs[0]);
engfunc(EngFunc_WriteCoord, maxs[1]);
engfunc(EngFunc_WriteCoord, maxs[2]);
write_short(999999); // life in 0.1 s
write_byte(255); // r
write_byte(0); // g
write_byte(0); // b
message_end();
I can't guarantee that either one will work for sure, but I feel confident about both of them.