You don't need a custom texture if you just want to change the size of the ubersplat. Export 'Splats\UberSplatData.slk' from War3Patch.mpq; open it in Microsoft Excel; copy the row that uses the texture you want; change its name to a unique four-digit code and set its scale to a 2 times larger value; save the file; import it into your map; change the path to Splats\UberSplatData.slk; select your buildng in Object Editor; hold shift while opening the 'Art - Ground Texture' field and enter the four-digit code of your custom ubersplat. This also allows all ubersplats to be used, even those that are unused (e.g. LSDL, CLTS, NMED) or intended for abilities (e.g. HMTP, UDSU, NVOL).