9/16 x 20 tpi spindles are common on most bikes, but you will find 1/2 x 20 tpi spindles on some, mostly children's bikes and others with one-piece cranks. I'd guess a 50-50 chance on yours being one or the other. Best to remove one of the pedals and measure. Keep in mind that the left pedal is removed by turning clockwise. Your pedals will not be metric.

Most bikes of that level will have those composite pedals. They are not high quality but they do seem to last a few years, even with heavy use. I'd recommend just using them while you're getting used to your new bike.

Edit: I have to say that even though I've installed hundreds of pedals over the last few years, I have not been able to eyeball a pedal to accurately determine the diameter. Two different sizes side-by-side, yes, but single pedals - no. You really need a caliper to measure, or if you have a 1/2" drill bit you can compare (or see if it slides into the crank hole - it should not if your pedal is 1/2").

Good luck and welcome to BikeForums