Which size would that be?
The sources I've seen for the Eclipse's length says that it's either 17.6km long or twice the length of the Executor. Since the Executor has three different lengths recorded, this puts the Eclipse at 16km, 25.6km or 35.2km.
An ISD is 1.6km, which is 17.6km when multiplied by 11... I assume you consider the 17.6km length to be the Executor's canon length. There are compelling arguments in favour and against this - being featured in the movies and being so very lightly armed for its size*.
Wookieepedia puts the Eclipse at 17.5km long, so it *is* nearly 11 times the length of an ISD.
*The Executor is reputed to be armed with something like 250 turbolaser batteries, 250 ion cannon batteries and 250 concussion missile launchers. This is a ridiculously low amount of firepower when compared with the ISD's 60 turbolaser batteries and 60 ion cannon batteries. Assuming weapon armanent is proportional to length(which it is not, width has to be factored in), 11*60 equals 660. Given that the Executor masses more than thirty or fifty times the ISD, it should be armed with between 1800-3000 turbolaser batteries, 1800-3000 turbolaser batteries as well as however many warhead launchers would have been seen as appropiate. Certainly in the four-digit range, if the Providence-class Carrier/Destroyer is anything to judge by - 102 torpedo launchers in a hull massing much less than an ISD.