I think he was asking about the drill size for the welding, but now that we're on the subject...
The correct spelling is Cleco, since you might want to Google it. It comes from Cleveland Tool Company and is probably like Kleenex and Victrola. The real name is something like "temporary wedge-type fasteners".
The color of the barrel denotes the size hole they fit.
The Old Pappies at Fairchild called them "buttons", which is actually a pretty good description of what they do.
Yes, you need pliers. I wish I had a nickel for every time I went home with my button pliers absent-mindedly left in my back pocket.
I'm also familiar with the screw-type buttons, both with wing nuts and hex-nut tightening. We had a special air drill motor for tightening the hex ones.
More than you ever wanted to know about Clecos, but thanks for reminding me of The Old Days.