I'm designing a tricycle and I wish to put mechanical disc brake calipers but I only got leverage of brake lever, I'm also searching mechanical advantage/leverage provided by mechanical brake caliper Also please tell what is the piston diameter provided by major mechanical calipers in the market
I assume that you're asking because you want to find mechanical disc brakes calipers that are compatible with your levers.
Generally, these brakes are simply marketed as MTB or road (also known as long-pull or short-pull). Typically long-pull levers are found on MTBs or bikes with flat handlebars and short-pull levers are on road bikes with drop handlebars. As long as the pull of the caliper matches that of the lever, it should work fine.
For example, Avid BB7s are available as MTB or road versions. The MTB version is long-pull and will work with any long pull lever. The road version is short pull and will work with any short-pull lever.