Size is also very important. It may not matter how large your avatar is while floating around in the skies of virtual worlds, but if it's too big, it won't be able to enter interiors - and it will look pretty silly. A fellow avatar suggested I should go for something in the human height range after I trying out an elephantine model. 1.8 metres was suggested as a good height, however I could not relate this number to that used by my software. If you have difficulty sizing your avatar, try importing the files of the avatars included in the Black Sun client software and using them as a reference for scale.
File size is also important.
The smaller the file, the faster your avatar can be downloaded and 'seen' by others. 30-40Kb is recommended. You can better achieve this by keeping the number of materials to a minimum. A fantastic tutorial goes through the process in detail, and is specific to Black Sun sites. It doesn't matter if you haven't got around to creating your own Web page yet, your VRML file doesn't have to be 'attached' to one in order to work.