Try an open-end wrench & see. That'll tell you the size, and if you try without pulling too hard, you can see if you can loosen the fittings.

With an open-end wrench it's easy to round off the corners of the fittings. That doesn't always happen, but when it does you'll wish you had gotten a flare wrench. The risk is if the fitting is tight you'll pull hard enough to round it.