OK - sorry it took me so long [I got caught up ina business meeting and then a fiasco with Griffgirls parts shipment with UPS] - anyway.
My FMIC is 32" x 4" x 12". Some of the 12" height extends behind the crash bar but only about the last .5" - any taller than that and it's gonna stick out the bottom and drag on speedbumps or stick to high and not provide proper cooling cause there is no airflow past the crash bar.
That 32" in length is from inlet to outlet and it could stand to be slightly longer if you wanted - I'd say you could get away with 36". The 4" depth is perfect and leaves plenty of clearance on either side but you could fit a 6" core in there if you wanted [Probably overkill on a modestly moded 3S tho - allow yourself some wiggle room to be sure so 4" not more than 5" say]
Hope this helps.