Does anyone have a good source or size chart for interior french doors (ie. closet doors). I think a 4068, 5068 and 6068 are pretty standard but I don't seem to see anything online suggesting any smaller sizes. I would like to try and use a smaller french door instead of bi-fold or sliding closet doors.
Those are pretty standard. Smaller dimension doors are "special order" that is why you don't see them anywhere. You will probably have to ask specifically for anything smaller. There are several door manufacturers. Might contact them direct and see what they can do. But like anything else that is special the price usually is higher than standard size.
https://www.google.com/search?sourceid=navclient&ie=UTF-8&rlz=1T4TSNA_enUS401US438&q=French+Door+sizes+available
Depending on your tools and time vs the cost of special order you can make them yourself. Our client on the last job made 6 raised panel doors while we were working on the remodel. He has a shaper and planer plus the usual assortment. I've made them with a planer and large router. It opens up the possibility of using unique wood. Two of the doors the client made were chestnut recovered from the floor joists we replaced.
How about sizes in between 4068 and 5068. I would like to call out a 4868 french door, basically two 2468 doors for a walk in closet. The 5068 is too big to fit the space and the 4068 will fit but bigger is better.