Dating back to 1991 when the first adjustable oars hit the market (my job at the time was to make them), but accelerating in the past few years, coaches and athletes have been experimenting fairly aggressively with oar length changes.
This practice has arguably been most radical among single scullers, who are in a position to try unorthodox rigging parameters that fit their body and rowing style without having to accommodate crew members, and anecdotally, among masters rowers, who are trying also to accommodate changes in flexibility, strength, resilience, and other factors.
I know of a couple very competent masters scullers who have tried some fairly extreme rigging numbers, and have reported both good boat feel and speed; one set oar length at 275, 10-15 centimeters shorter than the typical 286-290 range; while another overhauled their entire rig, setting oar length to 283, span to 154, and inboard to 87, and loved it.
As with most rigging changes, there may be unanticipated effects; in particular be alert to the change in the angle of the shaft to the water, which will affect the height of the handle. Many rowers have noticed their handle height is no longer comfortable after an oar length change; let's look at what is going on, and how to fix it.
In simple terms, as the outboard length becomes shorter, the angle to the water will be steeper, and thus the handle height higher. The solution is simple as well: change the height of the pin. But by how much? Some quick trigonometry gives the answer.
On sculling oars made by a popular manufacturer set at a length of 288 cm, the section of the shaft between the button and where the shaft joins the blade is about 150 cm. On my own boat, the distance of the pin to the water is very close to 30 cm, which makes the math extremely simple.
Sculling oar length set at 288
Using SOHCAHTOA, the sin (-1) of the angle where the blade goes into the water is 30 (Opposite) divided by 150 (Hypotenuse), or 2 (making the angle of the blade to the water approximately 11.5 degrees)
If you shorten the outboard of the oars - that is, the hypotenuse - by 2 cm to 148 cm for a total oar length of 286, to keep the same angle you get a new Opposite side of 29.6 (sin of .2 x 148).
So if you shorten your oars by 2 cm and keep the same inboard, you need to lower your height 0.4 centimeters.
Sculling oar length set at 286
It turns out that the insert on my own single is about 0.6 thick, which is close enough to 0.4 that dropping the pin one spacer felt perfect - though remember that height inserts vary considerably by manufacturer.
Note that I had been rigged a bit on the high side, so depending on your starting rig, you may or may not need to change your height. Anecdotally again, I know a few folks who are a bit overboated and as a result had their rig as low as it could go, and they found that shortening their oars made the geometry of their setup fairly uncomfortable.
Spacer width is approx 0.6 cm
On a sweep boat the numbers change a bit due to the much longer oar lengths; here is the quick math:
Outboard shaft length: approx. 205
Distance water to pin: approx. 34 cm
Sin -1: 0.166
Doing same math as above, shortening a sweep blade 2 cm results in needing to lower height by 0.3 cm - so the formula is more like one 0.6 cm height ring for every four centimeters of oar length change (again, remembering that height insert thickness varies by manufacturer).
tl;drFor scullers, for every two centimeters of oar length, change your height by 0.4 centimeters
For sweep rowers, for every two centimeters of oar length, change your height by 0.3 centimeters.