For 0.015 and under the minimum button I.D. is 8 x wire dia. There is no spec for O.D., Groove Width, or material. At a minimum the O.D. has to trap the wire. The wire needs to securely stay in place with out line tension. It can't be too large or the flying wire may become distorted during hook-up. On units and bellcranks I'm familiar with the buttons have been at least 0.250 O.D. There are also differences when using a button on a bellcrank or on a mono-line unit, where the torque wires have to be accommodated. Brass or steel are the materials of choice.