I have never tried it myself, but the obvious implication would be that the exchange of gases would be reduced roughly by half. The reason that capillaries are as small as possible, is that this permits the maximum possible diffusion of substances, particularly gases such as oxygen and carbon dioxide, into and out of the blood. Double the diameter of the capillary, and you double the amount of blood that is served by the same area of blood vessel wall.

On the other hand, if you did see fit thereby to limit the performance of the heart muscle, with larger capillaries there might be a small risk of blockage due to blood clotting and similar problems.

On balance though, I reckon that we need the highest practical density of capillaries of the smallest practical diameter, to get the best practical performance out of our hearts.

Keep well,
Jon