For most W123s the wheels were all 14 inch. There were, however, 15 inch wheels fitted to the longer chassis W123 and the vehicles designed for countries with very poor road surfaces.
Generally speaking it is harder to find the 15 inch wheels; and when you do they are likely to be a lot more expensive than the 14 inch equivalent.
The widths of the 14 inch wheels (fitted to the W123) were 5 and a half inches for the lower powdered vehicles (generally speaking 4 cylinder engines petrol - no EFI - and 4 cylinder diesel models). The more powerful vehicles more often than not came with 6 inch wide wheels.
A full listing of chassis type and width of wheel fitted is given in chapter 40-100 in the factory service manual (FSM)
The FSM can be viewed (legally and) for free at www.startekinfo.com
Click on resources then CD/DVD then pick the W123 manual. This site only really works well for internet explorer (which is a shame because Microsoft products are rubbish!)
These days there is generally speaking a better selection of 15 inch tyres available. Not many cars are made with 14 inch wheels that need tyres with high sidewalls any more. You can still get quite a good choice of high side wall high load bearing light commercial vehicle tyres but the ride quality isn't so hot for a car (so I'm told).
People on the whole seem to like the bigger wheel with lower profile tyre look these days so in some respects 15 inch is slightly trendier. There's not that much of a difference in sidewall height if you keep close to the equivalent rolling diameter of a standard 14 inch wheel and tyre combination.
Do a search for user MBeige and his 15 inch ATS "Bundts" (Bundts is what the Americans call what the Dutch call Kroonkurks which is what Mercedes call cast alloy wheel!)