Based upon what I've read here and in other posts, it sounds like you may be creating designs in Rocsim and then trying to build them. If that's the case, then you may want to tailor your future designs to more standard sizes. Just because it's in the RSim database doesn't mean it's readily available. I ran into that problem when I found online a file for a SM-2 missile, then discovered that it required BT-70's. Fortunately, I found that Jim Flis had some that I bought as part of a larger assortment (hey, no such thing as too many tubes!).
When downscaling/upscaling, builders will frequently use a rather odd-looking multiplier for their scale factor in order to get a result that matches the available BT sizes.
WW