Optimal tire size and pressure isn’t always known. Here is a great calculator I use when I set up bikes.

The “Bike + Rider” weight is usually about 10% over your out-of-the-shower weight, and includes your clothes, tool bag, bottles, bike, etc.

The middle calculator automatically shows front and rear tires separately. I recommend that.

For tire sizes, you can pick what you have installed, and see what’s the best pressure for front and rear.

I like to run 2 different sized tires so I can keep the same pressure front and back, but due to frame limits, my rear tire is still too narrow.

If ideal for a road bike is under 80psi, (light riders), then you might consider going with a thinner tire. You’ll find it easier to spin up.

If ideal is over 120psi, you might consider a wider tire, because higher pressures can be harder on rims, tire casings, etc.

If you’re riding dirt, then lower pressures are better, to a point. Under 30psi usually means rock crawling, low speeds, and a tubeless setup. Over 80psi usually means you’ll have traction issues on softer surfaces.

Also. this person’s domain is “Dorky Pants R US”, which is just really great fun.

http://www.dorkypantsr.us/bike-tire-pressure-calculator.html