There is no one size for each person, it varies from mfr to mfr. A 44 from Sidi is not the same as a 44 from Shimano.

There is no way to know if a saddle will work for you until you ride it.

Start at an LBS. Try on as many shoes as you want, you're not expected to buy them if you don't want them. But I would probably discourage you from doing that with the sole purpose of finding your size to then go online and find them cheaper. If you're going to buy them online, find an online shop that has a good return policy for bad-fits.

You should be able to get a good idea for size just trying them on, no need to mount your cleats until you're ready to put some miles on them. Some companies, like Giro, let you return them for up to 30 days.

I like to write down the brand and size that fit, even if I didn't buy it. It comes in handy if I want to try a different shoe from them in the future.

I would buy those items on eBay.