There’s a lot of variations and options here, but as a general rule you want to keep your stripes somewhere between 10 cm and 30 cm. If you make them too small they are more difficult to paint and lack impact. And, if you make them too wide they lose their impact and purpose and just end up looking like a large block of painted wall. However, the choice is yours, just make sure to plan ahead and sketch out your ideas to give you an idea of what you are going to get.
A tip if you want uniformity to your stripes is to plan for an odd number of stripes. This will give you the same colour stripes at both ends of a wall. This is the ideal choice if you are painting a single wall with stripes. However, if you are continuing the stripes on all walls use an even number of stripes.