Subdivided terrain for huge map?


In world machine, I realized a terrain of 10km by 10km. I used the tiled build option to have a field in 16 parts. The resolution is 505x505 by tile. With unreal, I imported tiled landscape. I settled the scale to 400 to have a terrain of 10km by 10km. But the terrain mesh is not important enough and the ground seems so sharp when I use the first person character. This is related to the resolution of 505x505 with tile ? Are there a way to increase the subdivision of the landscape ?

ps : sorry for my bad english