[jruhi4]

I'm surprised that in 11 days since you posted this,nobody's replied. I assume your IS is a sedan (I don't think you'd be so concerned if it were a Sportcross). Anyway, I just measured mine. Here goes:

The narrowest portion of the trunk (roughly the rear half) is 31.5" wide.
The wider front half is just under 53.25" wide.
The depth is 37".
The height from the trunk floor to inside bottom of the trunk lid varies somewhat,but 19.5" seems to be the shallowest point.

Hope this helps.