Minecraft wiki entry for Structure_Block, associated text: Structure Size
Enter the X, Y, and Z values to set the distance from the Relative Position coordinates. This sets the opposite corner of the structure, and defines its size.
Maximum structure size is 48x48x48 In Java Edition and 64x256x64 In Bedrock Edition.
When successful, generates a white outline surrounding the structure.‌[JE only] In Bedrock Edition, the outline is green, blue, or red (depending on the axis) and the preview design can be used to see every layer of fragmented structure and to change the structure size to be saved.