For beginners, you must first know that not all mats are the same. They are different for a reason. When people buy mats for the first time, they usually go for thicker mats. Thick mats are good but you need to make sure they are for yoga and not for regular workouts. Foam mats, Pilate's and Fitness mats are often confused with yoga mats. Fitness mats absorb impact while Pilate's mats are for catering to the spine. You don't want to use these for yoga. Foam yoga mats are available and are recommended for people with bad bones and joints. A foam yoga mat is also great for beginner's. The downfall, they are bulky to carry and make balance moves more challenging. They also prevent smooth movement between asanas. There are many different types of textured mats available for a stronger grip.
All mats are different sizes. The taller and broader you are the longer your mat should be. Plus, if you are short. Why would you want extra material to carry around?
Cheap vs expensive. Cheap mats for great for someone on the budget and for beginners. However, my experience with them have not been so great. They are PVC-laden (a toxic plastic with carcinogens in it & not eco friendly). Eventually, these fall apart, are not cushioned enough for the body, and can cause slipping and falling because of sweat. If you are a regular yogi and living the lifestyle everyday, I would recommend investing in a better mat. Don't be scared to invest a few extra dollars.
If you are looking to go eco-friendly then look for mats made from biodegradable, recyclable compounds like polymer environmental resin (PER) and thermoplastic elastomer (TPE). Cotton mats are also available even though there is not much cushion to them. You can always save your mat by using a yoga towel on top during your practice.
Overall, you have to find what works for you. It is like I tell my clients, "buying a mat is like buying a mattress". You want to make sure it compliments your needs. No one wants to sleep on a hard or lumpy bed. Your mat should be comforting and represent who you are. When you test your mat, you should test for portability, longevity, comfort, non- slip, cushion, and texture. With so many brands out there the decisions are endless.