@ Chance,
though I know little about Western red cedar the 4x6 dimension you have mentioned will be suitable for a small shed. Of course if you want to build a "traditional American frame" then this would not be the right dimension.
Depending on what it is that you will be building you might want to keep post/rafter spacing within a 42" range.
If the price is okay and the timbers are good I would purchase them and decide later what to do with them. Maybe a nice pergola. The possibilities are endless and only limited to by your imagination.
