Pythagorean theorem is your friend, assuming you have the dimensions of the Vee.
Measure the width across the top (widest part) of the Vee and draw a simple sketch. That distance (perhaps a little less to assure the round is sitting on 45º planes), divide by 2. Now use that number, square it, multiply X2, square root of the result, multiply by 2, that's your maximum. And it may not fit into a slide-in clamp. If the Vee blocks have that, measure across it's full inside radius and subtract a tiny bit.
Measure the width across the top (widest part) of the Vee and draw a simple sketch. That distance (perhaps a little less to assure the round is sitting on 45º planes), divide by 2. Now use that number, square it, multiply X2, square root of the result, multiply by 2, that's your maximum. And it may not fit into a slide-in clamp. If the Vee blocks have that, measure across it's full inside radius and subtract a tiny bit.