The patch that will be released on the day of launch of Mass Effect: Legendary Edition is apparently 11.8 GB in size.
That was leaked through the Sony servers and then reported by Orbis Patches. The patch can be downloaded from launch. According to the description, this is to stabilize the game and to clean up various crashes. It should also make improvements to the lighting effects and remove all kinds of small problems with the game. EA has not yet officially announced the patch.
Mass Effect: Legendary Edition includes remasters of the first three Mass Effect games, including all downloadable content. The collection will be released on May 14 for PlayStation 4, Xbox One and PC.