How do I work out the area or length of a wire when I only know the resistance/conductance and the length/area?

if cunductance = conductivity x area/length

then

length = conductivity / conductance x Area

i think, if the equation is transposed correctly, it’s been a while since school maths.

better check that one though.

You have given the correct equations.Lets take the case of resistance.

R=r*l/a
You know R and a ,so l=R*a/r

Here r is fixed for diffenet metals.So,you need to check out the r for the given material and go one with your calculations.