I would not use an excluder. Top bar hives vary in dimensions. I would try to have a minimum of the volume of a ten frame deep box. To calculate the volume (v) of a trapezoid cross section box:

Take the width of the top (a) plus the width of the bottom (b) divided by 2. Take that times the height (h) (right angle measurement from bottom to top). Take that times the length (L). hence:

((((a+b)/2)*h)*L)=v

A ten frame deep is 14.75* x 18.375" x 9.625" = 2609 cubic inches.

So if you figure the trapezoid at ((a+b)/2) * h and take that result and divide 2609 by that number you'll get the length of your top bar where the excluder would go. i.e.:

2609 / (((a+b)/2)*h)=L