If you don’t want to have any interference in your network, why don’t you simply simulate 0 rings of BSs (by setting LTE_config.nr_eNodeB_rings = 0)? This will create just one eNodeB in the center of your ROI. To also avoid interference among the 3 BS sectors, you might want to switch to an omnidirectional antenna pattern (LTE_config.antenna.antenna_gain_pattern = ‘omnidirectional’), which will then only create one eNodeB object.
Note however, that without interference you are often in saturation (regarding SINR) so your results might not be very meaningful.
This does not answer your question regarding the ROI, but changing that might be unnecessary then when only generating a single eNodeB. Please let me know if you still want to change ROI size and/or UE placement and assignment.