In Gaea check your build resolution. Unreal engine has specific map size settings. click on it and it should show you the unreal size to pick. eg: 2049 is 2017 for exporting to Unreal.
Cheers. I know I have to use a 1023 resolution as that is the max I can work with in the free Gaea. Then I use the typical math (X/Y = Map Size x 100 / resolution, Z = Max Height x 100 / 512).