Here is one description-

[You are in your room, which is obviously very familiar to you since it has been your room since you were a baby. There is a bed in the SE corner and a night stand directly next to it. On the E wall there is a dresser with a window over it and in the SW corner a standing mirror. In the NW corner there is a rather large wardrobe. There is also the balcony to the S and your door to the N.]

To me I like to make it detailed. But reading it I see all these cardinal directions that may confuse people. Do I even need to describe what is in the room if it has no [I]real[/I] significance? Would you assume there is a bed and a dresser and a night stand because it is a bedroom? Would it matter to you the placement of it if I at least said those items are in the room? Should I use terms like "in front", "to your left", "on the right", etc. so exits would not be caught up in all that?

And what about this-

[This is the Great Dining Hall of the castle. This is where you and your family, your Royal Guests and all those who live up stairs dine at. This room is very lavish. You and your parents, with the same sitting arrangements as in the throne room, sit closest to the E wall with their back to it in the middle with the Royal Guests next to either you or your father and then your Royal Family and then the rest generally filling up the table completely. You have waiters to serve those at the table and the finest of meals are prepared in the room to the N leading to the Kitchen. To the W is the Main Chambers where the throne room is at.]

Does half that even matter to you? Should I cut most of that and just say a dining hall with a few servants rushing back and forth and that is it?

And finally this-

[This is the Horse Stables. This is where all the horses are kept and cared for. Each horse has it's own lodging with fresh straw for bedding and food and water. The horses each and every day get a decent work out of running and training and a good rub down with the brush to keep their coats shiny and dirt free. The hounds and other dogs are also kept here and cared for as well. To the South East is the Northern Courtyard and to the South the Black Smith's.]

Does one assume everyone knows what a horse stable is and what goes on or should go on there? I like to give long descriptions, but I tend to over it do it some times. What do you feel is the best course of action?

If you fill in all the details about a room, the player can "see" it better, as if they are there...
But, too wordy, and some people will just skip it all together...
I would not want to see (read) a half screen description of the room every time I enter there...
So, put the long winded description in a {once:} Then have a short room description next.

I personally like a reasonable size description the first time I enter a location and thereafter just the bare minimum. So, the {once:} suggestion sounds like a great idea.

To me I like to make it detailed. But reading it I see all these cardinal directions that may confuse people. Do I even need to describe what is in the room if it has no [I]real[/I] significance? Would you assume there is a bed and a dresser and a night stand because it is a bedroom? Would it matter to you the placement of it if I at least said those items are in the room? Should I use terms like "in front", "to your left", "on the right", etc. so exits would not be caught up in all that?

I would only use cardinal directions for things that it matters - basically just exits (and personally I would not use abbreviations). I would assume it has a bed - so tell me what makes this bed stand out!

Some of your descriptions are what the player character knows of the room; this is stuff I would put in the {once:} bits.

Everyone knows what a stables is, so omit thayt sentence, but the rest is telling us the horses are well cared for, so do something.

Regarding your question of directions: "on your left", etc. are much more natural and vivid, but only really work if you know from which direction the player will enter, e.g. a room with only one exit. Also, depending on how your room is structured, it might imply the player just stands around there instead of moving in the room. Especially if anything is located "behind you".

You could get the best out of both by some coding work and adjust "infront" with "to the right" depending on where you came from.

And when it comes to the length of descriptions, i mostly agree with the comments above: Extensive on first visit, short after. Just make sure to include a look around command for forgetful players or those playing over multiple sessions.

Including something in the short description is also a great way to lead the player's attention and remind them of their objective, just think of this:

[The outside of the barn is stirring with heat even this early in the morning. The keychain still dangles at the weathercock.]

The inverse of this is also good to tell them something is up in horror games, but this is too off-topic here.