Question about Fractal Define 7 Black and White TG

If I understand you correctly, you want to use a watercooler with radiators mounted on the top of the case. With this layout RAM height should actually not matter and issues with space would rather be dependant on where the RAM slots on the motherboard you plan to use are located.

RAM slot locations on Motherboards are not always exactly the same but they normally only differ slightly. There are no exact measurements to this in specsheets or the likes.