I have a pair of original oak ones, but it's a little hard to get an accurate measurement of them. This is because the width varies along the length of the filler strip.
The length is about 27 1/4", and they are about 1/4" thick, but the width is a little harder to figure. At one end they are 1 1/8" wide, at 12" down from the narrow end they widen out to about 1 3/8", and at the opposite end they appear to be 1 7/16" wide. Of course, once installed they can't be seen, so the measurements are not all that critical. For all I know, the original ones may have varied some.
Note that the filler strip at the top of the picture below is broken at the wider end, and missing a small piece of the wood.