I'm not sure I agree with the proposed resolution. IMHO, the schema should be SQL standards / defacto standards based and not have any DB specific attributes. (FWIW, it appears that CREATE INDEX is a defacto standard... I'd describe a defacto standard as one shown to be supported by most of of DB servers Torque supports. )
With this in mind, the solution to this would seem to be:
1) Deprecate the size attribute (and remove it in 4.0?).
2) If this seems like a good feature to add into Torque, add support for it via that option element which is intended to support DB Server specific items.
But for 2) we'd have to think about how to support / test these DB Specific items prior to release, how to document them, and how to deal with possible DB Vendor changes between versions. So should this be Torque "core" or an "add-on"?