Here is some definitive information on what the stock sizes are and what the balanced upgrades are. Note, there are 3 different sizes of stock rear swaybars out there, excluding IPD and GT swaybars. The rear springs in a wagon are stiffer than a sedan, thus the wagon rear bars are 1 size smaller than their sedan counterparts. A GLT and a Turbo are identical in suspension and trim. The only major difference is the GLT does not have a turbocharger.

Swaybars for the Volvo 240 (not including GT):

Stock Wagon:
Front: 19mm
Rear: 15mm (and 'wagon' springs)

Turbo or GLT Wagon:
Front: 23mm
Rear: 19mm (and 'wagon' springs)

Stock Sedan:
Front: 19mm
Rear: 19mm

Turbo or GLT Sedan:
Front: 23mm
Rear: 21mm

Now, you want to upgrade your non-turbo Wagon to some better swaybars. You will basically want to give it the same setup as a Turbo Wagon. Thus, you will need a 23mm front bar off any turbo (sedan or wagon), and a 19mm rear bar from either a non-turbo Sedan, or a Turbo Wagon.

If you want to go a step further than the Turbo swaybars, now you should be trying an IPD 25mm front bar and either a 21mm Turbo Sedan rear bar or the 22mm IPD rear bar.

The stock suspension on all un-modified Volvo 240 cars has a slight tendency to understeer. This means that during a hard turn/corner your front tires will slip before your back tires, and this is a safe setup (more crumple zone in the front hood and engine than in a side door). IPD bars try to balance the suspension to a point where all 4 tires would start to break free and slip at about the same time during hard cornering. If you install a rear bar that is too large for the front bar, then your car will oversteer (back end breaks free and the car starts to spin sideways) and this is considered very unsafe.

If a set of IPD swaybars were dropped into my lap, I would probably sell them. A set of Turbo swaybars is just fine for my non-turbo wagon and my driving habits. If my local roads didn't have potholes and off-axis railroad tracks, I would consider keeping them because they really do flatten out the body roll. Since you've got them in your possession, give them a try and see how you like them. If you think that they're a bit too harsh, try switching swapping the poly end-link bushings for a set of Volvo rubber bushings ($1.99 each, you'll need 4 of them). If that doesn't help enough, try a regular set of turbo swaybars with fresh bushings. I would strongly advise using new rubber bushings on the end-links of any swaybar to decrease the initial shock of a pothole. Poly bushings tend to have more ride harshness, but do firm up your cornering just a tiny bit more. For more info on Turbo swaybar bushings, feel free to read the following post:
http://www.brickboard.com/RWD/index.htm?id=616911

FYI, if you come across a GT (78,79,80) in the parts yard, there are several possibilities as to what you might find. Some of them were equipped with turbo swaybars, while others used non-turbo swaybars and stiffer springs. From the few opinions which I've read that compare the 3 years, the '79 is said to be the best handling GT (and all of the 240 Volvos for that matter). For the '79 GT a special 21mm front bar was used along with a stiffer set of front springs, and a special thick 23mm rear bar. This is the thickest rear bar that you will find on a factory package. Other notable items to grab from a GT are the upper (and sometimes lower) strut braces, shock tower reinforcement plates (between the front shocks and the underside of the strut tower), special swaybars or springs if equipped, and the aluminum wheels. If you plan to keep the IPD swaybars, I would strongly encourage you to at least buy a set of upper strut-to-firewall braces. These were equipped on all the GTs and all the Australian Volvo 240s. I got a set of used upper braces shipped to me from a guy in Australia for about $35. (thanks Angus)
Much of the above information can be found on the sweedishbricks 140/240 FAQ. Open the following webpage and start reading from the 1975 model year:
http://swedishbricks.net/ModelFAQ/140240model.html

God bless,
Fitz Fitzgerald.
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'87 Blue 245, NA 225K