My son has a Pacific Cycle mountain bike that lost one of its crank arm nuts. How can I get a replacement or find out what size it was?

Any bike shop in the world should be able to give you several or call the Pacific customer help line in Madison WI and have them send you one. You can find their 800 number using Google. Roger

If it's a square taper bottom bracket, the nut is most probably threaded M8x1.0 mm. Any bike shop should have a bunch lying around from replaced bottom brackets.

The bike is strange... it has a sticker on the bike that reads Murray and yet there is a model/sn sticker that says Pacific... I didn't realize they were the same. The crank has a slot but the bolt is totally round.

That should be the cap of the crank. - that comes with the cranks, you can get another one only from the same crankset. But just as a dust cap there are a lot of types that fits.

If one of those caps gone missing it's no biggie, the cranks are still held by the bolt as it should. The caps are there just to make a finish with the rest of the crank to look good and to be dustproof.

Here's a good guide to dealing with a nut-type bottom-bracket: https://www.utahmountainbiking.com/fix/crank2.htm

The BB spindle is actually threaded at the end:


And uses a 10x1.25mm serrated flanged nut:


These actually work better than the higher-end fixing-bolt and washer combination. Even when insufficient torque is applied, the serrations on the nut keeps it from working loose and falling out.

Thanks for the correction. The threads I mentioned were for a bolted type spindle.

Importantly, the bike shouldn't be ridden if it's missing the nut (or bolt, depending on the BB).