Who does it seem that all the Starfleet ships we've seen, with the exception of the 1701 in TWoK, have vertical Warp Cores? On the Enterprise D it seemed to take up 2 decks, but specs, as I recall, showed it at 4 decks high. That's some 40 - 50 feet if you assume those decks it transverses have Jefferies tubes.
The warp core on the Galaxy Class is much larger than just four decks. Even the Intrepid and Sovereign class ships, both of which we have seen ejecting their cores, have warp cores that span more than four decks. The Galaxy class Warp Core spans at least 12 decks, and this doesn't even include the antimatter storage pods and the Deuterium storage tank.
The Intrepid class warp core looks to be about eight decks tall, while the Sovereign class core is roughly fourteen decks tall. Comparably, the Defiant class warp core is only three decks tall.
These figures were obtained, by looking at various MSD's of each ship class and simply counting the decks.
You can check out this guys blog: http://lcarsgfx.wordpress.com/
He has some great highly detailed MSD's for those that are interested. You can also find a few on Doug Drexler's blog.