Hello all, I'm making a table runner for an over-sized table and would like some help with the width measurement.

The table is 60" wide. And I'm not sure how wide the table runner should be so that it is in proportion.

The table is 90" long and obviously I'm making the runner that long and I'll probably put tassels on.
Thanks for your help!

I would try about 1/3 of the width to start with. You could fold a piece of fabric or newspaper that size and see how you like it and adjust as you wish.
I don't think there is any rule.