To determine the size of the curtain rod you need, you should follow these general guidelines:

1. Measure the width of your window frame or casing.
2. Add 20% to the measured frame or casing width.
3. Determine the appropriate length of the curtain rod by adding 6 inches to both sides of the window frame or casing.
4. Make sure the curtain rod extends three to four inches from each side of the window frame.

Standard curtain rod lengths range from 24" - 144", while mini and small curtain rods range from 12" - 48". If youre looking for a more custom look, you can measure the width of your window and add 4-6 inches on each side to account for the hardware.

Its important to keep in mind the type of curtain you want to hang, as well as your style preferences, when selecting a curtain rod. Additionally, you should consider the ideal length for different curtain fabrics and styles.