The bigger the bore, the more effort is required for braking. Personally, for a manual brake setup I'd want 1.03" or smaller (for stock calipers). A 1.125" requires a lot of leg. They often publish the bore size on OEM replacements. It's all relative to caliper bore displacement too. You may want to just try what you have and see how it feels.
If you need to change MC's, Dr. Diff can set you up with an MP aluminum one with adaptor for a pretty decent price. He can provide valuable advice on the subject too.