This is one of the central precepts of terrain design. When you work on a terrain, and especially when you export, the amount of detail in the final result is calculated based on the size or resolution of the export image. Even when you export as a mesh, the image resolution is critical.
In Gaea, there are two different resolutions you work with: the Preview Resolution and the Export Resolution.
The Preview Resolution controls the quality of detail of your terrain preview in the viewport. You can choose between 512 x 512 (0.5K) to 4096 x 4096 (4K) resolution in cubic increments of 512, 1024, 2048, and 4096.
The higher the resolution, the more detail you will get. However, larger resolutions can take 4x more time to process the image. Switch to higher resolutions only when you need to.
You can switch between preview resolutions as often as you like. When you switch the resolution, the terrain preview is rebuilt. For 0.5K and 1K previews, the process is usually fast. For 2K and 4K, it can take longer.
The Export Resolution controls the quality of detail in your built/exported terrains. In Gaea, there is no limit on maximum resolution except for what your hardware allows. By default, you can export to resolutions of 1024; 2048; 4096; 8192; 16,384 (16K); and 32,768 (32K).
For resolutions beyond 16K, we recommend using Tiled Builds to contend with unlimited resolutions on even modest hardware.
As with preview resolutions, high export resolutions can also take longer than the preview - something to be expected with builds. In essence, you create a "template" with your settings when you create a terrain at preview resolutions. When you "build" a terrain, Gaea takes that preview template and uses those settings to build the terrain from scratch using the full export resolution. This results in more detail than is possible in previews.
For more information on building terrains, see Build Manager.