You can choose from a pre-defined list of square tile sizes or specify a custom size. Please note that the larger the heightmap the greater the load on system memory. With very larges size heightmaps, you may suffer some performance issues. One option to bear in mind is that if you need a large heightmap it may be better to build it from sub-tiles. For example, if a 1024x1024 heightmap is needed, try creating four 256x256 tiles instead or use the windowing mode.
Predefined Size: this gives the user the option to select from a commonly used set of sizes
Custom Size: this allows the user to specify the height and width of the heightmap
X: specifies the width of the heightmap
Y: specifies the breadth of the heightmap
Lock Aspect Ratio: keeps the aspect ratio of the current heightmap
Current: places the current width
and breadth values into the appropriate X and Y edit boxes
When you resize the heightmap, you can specify a resize mode that resamples the existing terrain to the newly specified size. Resampling refers to changing the pixel dimensions (and therefore display size) of an image. When you downsample (or decrease the number of pixels), information is deleted from the image. When you resample up (or increase the number of pixels), new pixels are added based on height values of existing pixels. You specify an Resample Mode to determine how pixels are added or deleted.
These following options can be selected from the: