Large maps with 1m resolution?

I’ve created a 1627 x 1627 grayscale height map image which represents height data sampling every 5m. When I import it into Unreal 4 I use a scale factor of 500 on the X and Y axis to achieve what I believe is a real-world scaled map of the maximum possible size of 8129m x 8129m. I was hoping that the resulting landscape would have a resolution of 1m - i.e. the distance between vertices is 1m which allows a decent resolution for sculpting river banks etc. However it seems that the distance between each vertex is 5m which doesn’t give me much scope for accurate landscape sculpting. I’m very new to all this so I may be doing something stupid.

Could anyone advise me please on the best way to create the largest possible map size (while retaining 1m vertex resolution) using a 5m height map sample?