take the one with 7.1 cm - yes, not the XL or XL adventure - they are much too big
I have the 7.1 in my HundeTagi: Galgo, 19kg, Viszla, 24kg, Weimaraner, 28kg, Rhodesian Ridgeback, 43kg, Lagotto, 15kg .... fits all of them well, not too big or too small

I'd say the biggest. It's adaptable.
Personally, my dog (Maremma Shepherd - 40 kgs), the collar and GPS are totally suitable. For a border collar, I suggest using the largest one, as the collar is adaptable in any case.