Karting Gloves come in many sizes, but they are all pretty much for one thing: To protect your hands from various things.
Yes, for the most part, anywhere you race you will have to use gloves. And even in the rare chance you don't have too at a smaller track somewhere, it's recommended you still do. Besides you can get a quality glove even if you don't want to spend a lot.
You can find any type of glove to meet your needs. They make fireproof gloves, or just plain gloves that basically will give you better grip on the wheel and protect you from a scratch.
Those seem like easy enough materials. Now, all you do is wrap the tape measure around your hand across the top of your knuckles with your fist clenched. Write that measurement down. I can't give you an exact size for what your measurement is because each brand of racing glove goes off a different chart for their sizes. Look up the brand you want and find their sizing chart to reveal your correct size.
The more you spend, the more comfort you get for the most part. There are exceptions as fireproof is more money. But for the most part you will get an increase in comfort, or padding, or grip. Leather and a stretchy type material are usually what they are made out of. Make sure the grip portion is not too slippery, or not there because a lot of karting steering wheels are smooth aluminum and you will need grip on the gloves.
Most gloves will fit up your wrist, with a velcro strap around it. This keeps the gloves firmly in place and they won't be coming off anytime with these. Try your best to find somewhere you can buy them where you can try them on, as everybodys hands may be different. The size measurement is only the first step in finding the right glove. You want them to be snug, but not too tight that your hands feel like they are getting choked.
Most racers don't concern themselves too much with the gloves, but they are important. If you race in anything that has a higher risk of fire, make sure to get fire proof gloves. Otherwise it's up to what you find the most comfortable.