I have not bought fabric online for these patterns because of this very issue...how much fabric do I need for the repeats plus some extra for "show" to see what the original fabric was. I think it would be super helpful to include that info before you click on a fabric such as being able to do a search for OBW in general.
The OBW book basically says 6" to 8" repeats will only make a very small project and more or less suggests doubling the repeats ie cutting them every 12" to 16 ". She basically suggests 24" repeats with 6 for hexes at exactly 4 yds, plus adding an extra yd or half yd = 4 1/2 to 5 yds total. Octagons she suggests 8 repeats at 24" = 5 1/3 yds exact or 5 3/4 or 6 yds with extra.
Hope this helps.