Open the corresponding dominion management page, click on [SIZE INFO] to view the current dominion size information:

To modify the dominion size, click the [RESIZE] button on the size information page, which will open the size modification interface:

For example, if you want to extend the dominion 10 blocks to the north, click [EXPAND] next to North (Z-), then input the number 10 in the opened interface:

Then left-click the green concrete in the center at the bottom to complete the expansion.