I've got a set of the ones you linked (at least looks identical) and I just managed to squeeze two wires of the 16 AWG silicone wire I have through. The opening is about 5 mm in diameter.
They're not the greatest quality though, if you're investing in some nice silicone wire might as well buy nicer quality plugs as well.
Edit: I might add that the wire I have at hand is not proper test lead wire, just some random silicone wire from Ali/etc. The wire diameter is 3mm, the opening is a bit flexible.