John,
can't tell you on the wire.
if you know the actual antenna diameter, you can divide it by your scale (35?).
so 1/2" = .500 / 35 = about .014, etc
on the sprue, i do mine over a candle.
hold the sprue by both ends and turn it between your fingers.
when it gets shiny and a bit soft looking, pull the ends slowly apart.
practice and you will get the hang of it.
it is easier than it sounds.
and they give you plenty of extra in each kit! :-)
but you can't do it twice!
once you stretch a piece, that's it.
use it or throw it out, but you can't 're-stretch' it.
i tend to 'eyeball' the sprue diameters.
when i use styrene rod, i multiply its size by 35 to see if the size makes sense.
ed.