In building our pond how do I determine what size bentonite pellets or powder)would mix best with my soil? Thanks

Assuming you have done a bucket test and know how much bentonite you need. I just use 3/8 inch pellets which where easy spread with a fertilizer speaker and little dust. Powder bentonite has a lot of dust when spreading it and I don't think it will do as good of a job. After spreading the bentonite I tiller it in the soil before packing it. I did thIs in two layers

Yeah, stay away from the powdered stuff.

Thanks for the help, much appreciated.
JCH

If possible, try and get a sample of the bentonite before buying. Put the sample in some water and see if it swells as advertised. Not all bentonite is created equally.