I have Ken's Bolt On Grab Hooks on my bucket. Great product.
Ken only offers 5/16" Grab Hooks because 5/16" hooks are a sort of universal size.
5/16" Grab Hooks will hold 3/16" (sort of), 1/4", 5/16" and 3/8" chain securely.
5/16" Grade 43 chain is likely what you want. With a compact tractor you will lose wheel traction before you can stress 5/16" chain.
3/8" chain gets awfully heavy working with it during a hot afternoon.
Both Lowe's and Home Depot offer a box with two lengths of 5/16" Grade 70 chain with 5/16" Grade 70 Grab Hooks attached to both ends at a very good price.
(Grade 70 chain = Transport chain)
Even adding in the cost of two clevis Slip Hooks to replace two of the Grab Hooks these kits are a bargain.
If you will use your bucket to "roll" saplings out of the ground, roots and all, you will want 6' of 1/4" chain with a 1/4" clevis Slip Hook on one end. The small chain will grip 1" - 2" trunks where larger chain will slip.