To get back to the topic of Interdictor-ships (and then I'll shut up about it) I don't think, that we can use the Peregrine / Modified Strike Class as examplary for a Strike-class-sized design with gravity-well-projectors. The gravity-well-projectors in that particular case were salvaged from a captured Immobilizer-418-cruiser and not originally designed and developed to be used aboard a smaller vessel.
Following I have a number of quotes and descriptions found on the Wook in regards to Interdiction-ships:
First the Detainer CC-2200
http://starwars.wikia.com/wiki/Detainer ... or_cruiser
CC-2200s were predecessors of the CC-7700 frigates, and the main interdictors of the Imperial fleet c. 19 BBY.
Nothing on size or armament, only that they are the fleets main interdictor in 19 BBY so lets have a look at the CC-7700 frigates
http://starwars.wikia.com/wiki/CC-7700_frigate
The warship was based on its predecessor, the Detainer CC-2200 interdictor cruiser. A product of the Corellian Engineering Corporation, it was described as an "older sibling" of the Corellian gunship. The CC-7700 was designed to serve as a platform for a gravity well projector, and was only lightly armed.
Again, we only learn, that the ship was only lightly armed. However it is also described as "older sibling" to the Corellian gunship, suggesting the CC-7700 as well as its predecessor Detainer CC-2200 are in a similar size-class (which would be 120 meters).
However, if we use the search-function on wookiepedia, we get a second entry for the Detainer CC-2200
http://starwars.wikia.com/wiki/Detainer_CC-2200, which tells us, that
The Detainer CC-2200 had stubby wing like projections, each of which housed four gravity well projectors which could be used to pull vessels from hyperspace. The cruisers were thickly armored and had downsloping bows.
Again we learn nothing about its armaments, but that the vessel houses four gravity well projectors (not one or two) and that the vessel is thickly armored.
Since there were two entries for the Detainer CC-2200 I took another look and also found a second page dealing with the CC-7700, in this case on upgrade to the original version called CC-7700E (suggesting there might have also been an A,B,C and D version, the Enterprise sends her regards)
http://starwars.wikia.com/wiki/CC-7700/ ... on_cruiser . This one is described as
Compared to the original CC-7700 model, it had upgraded armor and weaponry. Its six shield generators were complemented by a carbon-nanofilament armor hull. Defensive weaponry included eight quad laser cannons upgraded to be almost as powerful as Clone Wars-era turbolasers and a large number of anti-starfighter cluster bombs, designed to go off in case of the proximity of starfighters. It was thought that even if a TIE Fighter could slip through the defenses of one of these ships, it would need to travel at almost its full velocity to be able to pierce the augmented hull.
The ship could project a gravity well several hundred planetary diameters. Numerous cruiser's fields would be overlapped to create an even larger interdicted area.
Again nothing on size, but the vessel seems to be a powerful little bugger with six shield-generators in addition to the gravity-well-projectors and eight quadlaser-cannons comparable to clone-wars-era turbolaser-cannons. Also we are given an idea about the range of the gravity-wells the vessel projects.
As a last and for completeness sake the entry on the Agave-class picket ship
http://starwars.wikia.com/wiki/Agave-class_picket_ship . While those were a NR-design of 16 ABE, it is also suggested, that at least some of them had interdiction-capabilities.
Agave-class pickets were equipped with Class 1 hyperdrives, powerful sensors, a good stealth package, and shotgun hypertransceivers which allowed them to report information back to the battlegroup with a minimal risk of detection. Some also apparently have interdiction field generators, as was used by the armada chasing the Teljkon Vagabond. In the event of detection, an Agave-class ship could quickly jump into the safety of hyperspace. Agave-class picket ships' sublight engines provided moderate speed, but were capable of dealing with enemy patrols.
Considering the lenght of the Agave of 190 meters, this might come as a surprise, but is still better than the comparison to the 120 meters of the Corellian gunship.
Based on all of the above I would guess the Detainer-series of Interdictor-vessels to fall in a size-range between 200 and 400 meters and to consequently be smaller than SFS's Immobilizer-418 or Loronar's Strike Cruiser.
Their Corellian origin is also noteworthy given the propensity for Corellian designs to be used by smugglers and pirates.
Considering, that a major share of the workload of Interdictor-vessels would be the hunt for criminals such as smuggler and pirates and in most cases against those the use of a Star Destroyer or 600 meter long cruiser would be overkill the need for a smaller Interdictor is a given.
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