1. A thick curtain or a thin one?
That depends on a few things.
Ask yourself the following question:
Is there anything to block the sunlight?
Too much sunlight during the day can over heat the house or cause damage to the furniture. If the answer is no then definitely a thick curtain. Look for the ones which have a lining of a sun blocking fabric. If the answer is no then either thick or thin will work. If you have a rolling blind it would look nice to have a thin one with some design on so it will contrast the blinds behind it.
2. Size.
Width!
A. Measure your windows properly. This will help the sales person give you a better estimate on the cost of your curtains. Measure the height and width of the window/door including the frame. If you already have a rod or a rail from where it will be hanged include that as well and clarify it to the sales person.
B. If you are deciding on your own about the size then here is how it goes:
If for example the width of your window is 1.40 m you add extra 40 cm which will cover for the rods which are always wider. Therefore your total width is 1.80 m.
For a thick curtain you multiply by 1.5. We do this because we want the curtain to cover the width of the window and have some pleats. Too many will make it look over the top. Therefore you will need 2.70 meters.
For a thinner curtain you will multiply be 2.5. Thinner curtains need more pleats to look rich and full. Hence you will need 4.5 meters.
Height!
You will also need to be careful when looking for the height of the curtain. There are fabrics whose height is 1.40 meters. If you want a taller curtain then inevitably the seamstress will have to saw 2 pieces together and that doesn't always work out nice. So, look for fabrics that are 2.80 or 3.00 m or more to cover not only the window's height but also the extra 5-10 cm needed for the hem at the bottom.
3. Color!
Firstly think of the other items in the space. Is the sofa / bed loaded with color and texture? If so then go for something simpler. The same goes if the other items are minimal or plain. Dare to do something extraordinary in the curtains. Also take into consideration if you want a more "heavy" classic design, a simple minimal or a bright monochrome.
Hope I helped!