|Scaling is somewhat arbitrary. In principle you just have to know how the mapping is between your 3D program (Blender) and the target environment (OpenSim or any compatible online world). Then all you need to do is to work in the correct scale. You even can later rescale your model in your target system.
So, what is the issue with blender ? …
It is all so simple that the Collada Exporter for blender assumes that one Measurement Unit is mapped to one real life centimeter. But the common expectation in OpenSim is that one Measurement Unit is mapped to 1 Meter. Unfortunately this missmatch very often leads to the import of “tiny objects”. The common fix is to scale up the models in Blender by a factor of 100.
But there is a nicer fix, which adjust the Collada Exporter to behave according to our expectations. See this video above to find out, how you can fix the Measurement missmatch between Blender-2.49b and your online world once and for ever.