I think they are typically the same size, but no one says you have to be typical. If different sizes look good to you, do it.

I like the effect of different widths. Personal preference i think.

personal choice in my opinion

Yup, no rules make it however you like it or want it or if you are short a color by all means make a few skinnier strips. I actually think it adds visual interest to make them different sizes.

They don't even have to be all straight either, you can have strips narrower at one end than the other.

I agree with others that there's no absolute. Of course, REAL piano keys better be the same size!! (laughing...)

I did a piano key border on my teahouse dragon quilt. see (I need help) thread I did on that project. I did it with all the same size pieces. Then again I am one of thoes that will adjust the towels on there racks if one is out of place.

Ray..

I've made a ton of piano borders, I perfer varied.
Have fun, post pictures.

I like the varied widths. Gives it personality.