Brand new bike. Briefly removed the rear disc brake caliper, now it's too tight
As mentioned in comments you most likely applied the brake without the disc between the pads. The pistons advance and do not return all the way to their original position - this is how hydraulic brakes automatically adjust for pad wear.

The pistons can be pushed back in. Remove the pads then use a tire lever to push the pistons back in. Then replace the pads. You should be able to find numerous videos showing you how to do this on YouTube.

You should check your disc is not warped, and then reinstall the caliper unit. Again there are plenty of how-to videos explaining how to align the caliper. One way is to loosen the caliper bolts, apply the brake so that the pads grip the rotor them tighten the bolts, then release the brake.