Canadian wall builder Magic Lab supplied us with these Industrial Grade 8 Steel zinc coated round base t-nuts. These are thicker than normal t-nuts.
The barrel is 1/2" long.
These work with Standard 3/8" x 16 thread climbing hold bolts.
Recommended bit to drill the hole: 17/32" is best, 9/16" will work.
These do not come with screws. They have a countersunk screw point so will take many common countersunk screw heads and can be used with phillips, torx, or your preferred screw head style. The screw size and countersunk bottom of the screw are the important parts here.
Recommended screws: #10 countersunk head wood screw x 3/8-1/2" (for 3/4" plywood)
Basically, about 1/2 the depth of your plywood is recommended for fasteners, so if you get super thick plywood you can accommodate longer t-nut screws if you want, (if you do use longer screws, make sure they do not cross into the bolt channel).
The main purpose of the t-nut screws is to keep the t-nut from pushing out the back of the wood or spinning when you are bolting holds into it or climbing on it, and we cannot provide exact advice for your situation as situations will vary, so please take care when installing and consult a professional if you have questions.