The Pioneering MB pamplet recommends 1/4" rope for spars under 6" diameter and 3/8" rope for larger than 6". For added strength; when using sisal instead of manilla (sometimes hard to find); or when things will take a lot of stress and strain (never under estimate the abuse a group of young boys can exert on a project), I'd add an 1/8" to those rope sizes. We use mostly 3/8" manilla except when the timbers get larger than about 6" we go to 1/2" or even 5/8" for something like the center shear lashing in a Monkey bridge support. Larger than this is strickly used only for spans such as a monkey bridge main line but never for scout type pioneering lashing. Hope this helps.