If you don't install the plugs first, 5/8" should be enough. If you install them, and stagger them through the hole, maybe 3/4". Standard F-connectors are 7/16" hex, but because of the flats, 7/8" is too tight for side-by-side.

If you want a tight oval hole, try a 3/8" bit and oval the hole by applying sideways pressure or wobbling the bit. If you can get to each blocks face, you can use a chisel to elongate a hole.