How to increase the number of tiles in my map

I am sure this is super simple but I have been trying to figure it out for a few days, I am new to unreal and I am importing my height map on a large scale but I can’t figure out how to increase the number of squares to increase the definition its always stuck at the same number (the grey and light grey tiles that make up the map). when I make my map smaller the definition increases but I want to make it bigger and then increase the quality