Need advice on wire size

Would the #2 be big enough if you added the AC? How likely are you to add the AC? Only you can answer that. I don't know if it would pass code to wire the AC so it couldn't run with the air compressor or the welder running. A simple transfer switch would work for that. By the way, do you have an exhaust fan to get rid of the welding smoke?

I bet you would never have a problem with 60a out there but #3 and your 100 will work fine.. Most people build these 100a garage services and never use more than 30-40 at any given time.

One last thing, with a measuring wheel it actually added up to 136 feet. How much extra wire would you get to ensure everythign is covered? And extra 10 or 20 feet per wire?

Either size will fit in your 1-1/4" section of conduit, so you don't have to worry about that. Since you're never going to max out your conductor rating, the #3s will do just fine in your circumstance.

Big wire is expensive, I measure carefully and try not to get more than a couple feet of extra unless I have something else that I can use additional wire for. It's a lot more expensive if you come up short and have to buy a whole new run of it though so emphasis on the "carefully" part.

Plan A How much conduit did you install? Add in the sweeps, conduit bodies and a few feet for the terminations. Then add a few more feet just to be safe.

Plan B is to suck a string all the way through the pipe, pull up enough for the terminations and a little extra and measure the string. When you are ready to pull the string out soak a rag in wire lube and pull it through with the string, along with a sturdier pull rope behind that.

You could pull a string or rope into the conduit, mark it where it reaches the terminal to connect the furthest away feeder wire.then pull it out and measure it. (You measure from panel #1 terminal to panel #2 terminal, you do not measure from conduit end to conduit end.) You would then add 12" for every sweep, 6" for every 45, for each LB, pull box, junction box or panel box, add 2 times the depth of box/fitting into your conduit run measurement. Add it all up and this will be the exact length you need for each run of wire.