Geological models of fluvial reservoirs are often constructed with object-based modellling techniques. This approach requires a specification of the distribution of channel sizes determined traditionally from core and well log data. The problem with these data is that they only show the apparent thicknesses of the channel objects: sometimes the thicknesses are too small because the well intersects the channel at the edge and sometimes the thicknesses are too large because channels are stacked. This paper proposes a new approach to determine reasonable fluvial channel sizes with indicator variograms. The relationship between the channel geometry and the variogram is established. The relationship is verified with synthetic examples. The calculated indicator variogram range is shown to be insensitive to channel width and width/thickness ratio. Two real reservoir examples show how the approach works in practice.