I agree that size is important, but much of the whiskey flavor in the whiskey comes from the oak. With my new oak barrels, most early judging comments on non sour beers regarding oak (as in an oak aged stout or IPA) refer to the relative amounts of bourbon in the beer. A freshly used whiskey barrel has a distinct and strong oak character and I would recommend (if volume is not prohibitive) to do something like a bourbon barrel stout as a first beer and then follow with flanders. If the barrel is a 50 gallon + whiskey barrel, the volume issue is less important as surface area to volume is much lower. Your first flanders may have an overly strong oak flavor. My 4th batch in a solera (50% each pull) after 2 years in a 10 gallon barrel is still a fairly strong oak flavor. I just pulled all the beer and am cleaning prior to a new batch of flanders. My 15 gallon flanders barrel has a much more subdued oak flavor after less then a year. The smaller your volume, the stronger I would expect the oak flavor to be.