What size board should I choose?
As a guide I've always said if you have a big room put big boards in it, if you have a smaller room use a smaller board or consider a 2 or 3 strip floor. The reason for this is because a big wide board in a smaller room will have the visual effect of making the space look smaller. This is easily explained by looking at a typical hallway scenario where the front door leads into a hallway with an open space of circa 2 mtrs wide and 2 mtrs deep, then a staircase comes into the hallway on the right side and the hallway narrows down to 1mtr wide leading through to a lounge or kitchen etc. Let's say you choose a 260mm wide board and install it running away from the front door and down the hallway, at the narrowest point your hallway is 1mtr wide and this means you will have less than 4 full boards across the width. Psychologically you might count the boards when you enter the front door and when you only see three full boards and one trimmed board your mind may think “this is a narrow space” rather than admiring the beautiful 260mm planks. Conversely if you chose a 130mm wide plank (which is still a beautiful sized plank) you would have 7 full boards and one trimmed board which will look much more in perspective and not a number you quickly count hence the eye is drawn to the lovely floor and the space feels larger. Having said all this there is nothing wrong with very wide boards in a narrow space and your design and taste are a personal thing, there is no right or wrong.
Which way should I lay the planks in my room?
This is a very good question and again is really down to personal opinion however we have a few tips from previous experience. As a guide we say install the planks parallel to the longest wall; away from the main source of light or in the direction that you will normally walk in a room. This may sound a bit confusing so I will try to explain.
The first point is simply a perspective opinion, if a room is 4mts by 6mtrs we would normally advise that you install the floor planks parallel to the 6mtr wall. However if the room is laid out in such a way that the only windows are on the 6mtr wall and this happens to be a large 5mtr wide bi-fold door overlooking a beautiful view then you may be inclined to lay it parallel to the 4mtr wall because the light will run into the room and down the planks rather than across them and it's likely you will set the room out to look through the doors and hence down the boards. Visually a wooden floor often looks better when the main source of light shines along the plank rather than across it.
Finally you may want to consider the way the space is used. If it's a long corridor we would advise that you run the boards in the direction which you will be walking, it just feels much better to walk along a plank rather across a plank and in this case the direction would take preference over the light source.
Again I want to reiterate there is not strictly a right or wrong, it's your floor and your space but I hope this helps you make a good choice.