Antenna size alone shouldn't make them particularly hard to use in space. While there are forces acting on the antennas from the fields' interactions with deflected particles, it's not like trying to move a square mile array at high speed through a dense medium like the atmosphere (which of course is not a thing we do on earth - transmitters are static). Submarine receiving antennae are just great big long straight wires which would be even easier in space.

Mass will be a factor given their probable need to handle high current loads; packing and unfurling seems to be a thing we can do quite well; stabilizing them and keeping them in the correct orientation as the ship accelerates and the fields deflect particles could be really hard or quite easy, I have no idea.

In space you need no supports. A 2km dipole using essentially two conductive mylar 'tubes' inflated with a small volume of gas could easily fit within the RedDragon's trunk.

You might not even need the gas, just rotate them around the center and they're rigid.

True, although that would limit which polarization options were available to you. I am also presuming that the inverse square law is helping here in that you would not need the kind of signal strength the antennae on Earth need to get through the ocean, rather just enough to create a Dragon sized bubble impenetrable to ionizing radiation at a distance of a couple of meters.

What if it gets clipped by something or exposure to the interstellar environment causes a thin wire to become more brittle?

It's not really practical for a tiny craft.

Well, in space perhaps thats not so very difficult if not accelerating. Certainly the ISS could do that I think?