What makes a guitar a good beginner guitar?

Every guitar is different. Every company builds their guitars to their own standards and designs. Some companies have main focus points like "the thinnest neck" or "the lightest guitar". I'd have to say there's really no such thing as a beginner guitar or a professional guitar... just the cost and quality of the instrument. Some people prefer a thicker neck if they play a lot of open chords while others prefer a thinner neck if they play faster. Some players prefer a guitar with a larger cutaway on the body to access the higher frets. Some people don't care so much because they never play past the 12th fret. Honestly, picking a guitar is a very personal decision. What genre of music do you wanna play? How much do you wanna spend? What type of playing style is it? Certain woods accentuate certain sounds. Mahogany gives a lower deeper sound while maple would give a snappier brighter sound. Certain pickups are designed for a jazzy rock sound while others like EMG's are designed for intense levels of distortion. A beginner can learn on a $5000 guitar or a $50 guitar. The sound and quality will usually be reflected in the price. Someone who shreds and can play almost anything can make a $50 guitar sound like a $5000 guitar. In my experience i've found that cheaper guitars can cause bad playing habits to form and make playing a lot less fun. Of course you don't wanna pay a grand for a guitar if your not sure if you'll stick with it but ultimately... the choice is yours.

There really is no such thing as a good beginner guitar. A good guitar is a good guitar regardless of your skill level. So take neck size for instance since you mentioned that. The neck size you choose depends on the size of your hands and personal preference, not how long you have played. When someone says a guitar is a good beginners guitar they basically mean it's a good guitar for a low price, since beginners usually aren't buying $1000 and up guitars, but still a $3000 Les Paul is just as good of a guitar for a beginner as it is a pro if you can afford it. The reason most say not to spend that kind of money is because if you don't stick with it you've made a huge investment for no reason.