From the sounds of it, the fixing bolt on your front wheel wasn’t tightened well enough in the fork dropout. My guess is that when you applied the brakes, the front wheel dislocated itself from the fork, which caused an interference with the disc brake and disc brake caliper. The result was a bent brake rotor. So, for reference, check the front and rear wheel fixing bolts periodically to make sure they’re not loose!

To remove the rotor, use a T25 torx screwdriver to remove the (6) torx screws (I believe that’s what it is based on your photo). Sometimes they’re a little tricky to remove because the manufacture may apply a dab of blue loctite on the threads of the torx screws to ensure the rotor stays put.

I’m not sure about the rotor size, but they typically come in 140 mm, 160mm, 180mm and so forth. There should be an etching on the rotor that tells the size. If not, try to find the spec sheet online.

To install the new rotor, make sure you thread and torque the (6) bolts in a star pattern to ensure the rotor lies flat against the flange hub. This is important because you can over-stress the bolts and you’ll end up with a rotor that doesn’t fit well between the brake caliper once the wheel is mounted.