I have a 48" light duty
grapple on a 41hp tractor with a higher capacity FEL than the Kubota. I'd recommend a 48" for general use on any tractor up to about 50hp. If you are really just picking up small short sticks or construction debris then a 60 inch or larger might make sense but there are downsides to larger grapples. Many people make the mistake of thinking that a
grapple should be the same width as their loader. Not. Narrower means more manuverable, lighter and allows you to put more force into a single spot.
My
grapple was made by Millonzi who appear to be out of business now. WRLong makes and equivalent and Markham has just started making an equivalent too. The Markham looks pretty much like the WRLong but you can buy it direct rather than needing to go through a dealer so it will save you several hundred bucks at least.