Okay so a common thing I hear in boxing is stand shoulder width apart. What is exactly meant by this? Stand shoulder is this vertical wide or horizontal wide? I guess what I mean is have my legs spread more apart side to side than my shoulders or more front to back spread apart from my shoulder width?
This should be meant as horizontal, meaning spread your legs side to side where your shoulders should be or a little more apart. This is to help with your balance to not easily fall ever when getting hit. Do you go to a gym? I'm sure your trainer can help you more. (or youtube)
your feet must be under your shoulders for the best balance, is that vertical or horizontal wide depends on guard you are in, distance between you and your opponent, fighting style etc..
I think you are overthinking it a little man. Just shoulder with apart. Obviously you have to factor in your guard and things of that nature but it means just should width so you have the most balance. Don't try to overthink stuff.