I'm getting my downstairs bathroom done. Replacing the mixer shower. The system is a gravity system, cold water tank in the loft, boiler and hot water tank in airing cupboard on 1st floor and the shower is on the ground floor. The current 10 year old mixer works fine pressure wise and is a 15mm inlet on the instructions. Shopping around for showers it seems some come with a 1/2 inch inlet/outlet and some with a 3/4 inch one. Does it matter which one I get? Do I need to match them to my current pipes? If so the picture below is taken from the extension loft space directly above the shower. Some are big some are small! Any clues what pipes I've got or do I need to wait till my fitter has ripped out the old bathroom. My Mrs already moans about the shower not being powerful enough to wash her long flowing locks so want to max out what pressure we do have!!