Hope you don’t mind if I jump in…
first, you will need collada, →
(COLLADA) but I can't say to much about it, I just don't have a lot of experience with Collada, aside from research.
Now, as for the scale,
Avan’s right about simply scaling down the model, if it's to big,
however it may not be big enough, because your going from (Blender to Second Life) when you need to be going from (Blender to Collada to Second Life)
In blender and most other programs, "one unit, is equal to one meter" (39.3700787 inches),
but collada uses "one unit is equal to one centimeter" (0.393700787 inches),
so you may need to scale your model up, 100 times normal when importing to collada, unless you edit the collada import script.
just be sure to apply the scale transformations in blender, once all your scale changes have been made,
not applying the changes may or may not cause errors when you export, it’s just easier to apply the changes and not worry about it.
And it may also help to take a look here. →
(Exporting a mesh from Blender)it talks about importing blender mesh avatars into SL, and addresses the scale issues.
And finally, I don't know for sure, but I assume that the bones in the rig would need to have specific names in order for the SL animations to work,
maybe that's something collada helps fix, I'm not entirely sure,
But this page has some info and a download that may help. →
(Uploading and wearing a rigged mesh)Now keep in mind that I've done research into making avatars for SL, but never actually made or imported anything,
And a lot of my info comes from the SL wiki.
Beyond that, once you start working outside of blender, there’s not much else I can help with,
my knowledge outside of blender is a little sporadic, so I'm sorry if the info I've given is also, a little sporadic.
But if there’s anything you need to know about, in regards to working directly with blender, feel free to ask.