What is the difference in size between the small and large cradles?
The small cradle is about 16" and about 24" for the large one. Typically large butternut squash and pumpkins and medium to large watermelons work best in the bigger cradle and acorn squash, cantaloupe, smaller watermelons etc. work out nicely in the small ones.
Are the small cradles ok for watermelon or should I get the large cradle.
If you are growing traditionally large watermelons the larger cradles are better.
How much weight will both the large and the small melon cradels hold?
Both the small and large should hold over 50 lbs.
What is the easiest way to attach these to a hardware cloth fence? Should I use a D-clips, or is there a cool way to tie them that I haven't figured out?
Hooks or zip ties will support heavy melons. Those are the main options, but as you alluded to, there's a lot of options also probably laying around the house!