The distance between the endpoints of a diameter of a circle is equal to the diameter of the circle.

Given points P(-6, 1) and Q(6, 6), we can use the distance formula to find the length of the diameter:

Distance = \(\sqrt{(x_2 - x_1)^2 + (y_2 - y_1)^2}\)

Distance = \(\sqrt{(6 - (-6))^2 + (6 - 1)^2}\)

Distance = \(\sqrt{12^2 + 5^2}\)

Distance = \(\sqrt{144 + 25}\)

Distance = \(\sqrt{169}\)

Distance = 13 units.

Since the diameter is twice the radius, the radius of the circle is half of the diameter:

Radius = \(\frac{13}{2} = 6.5\) units.