Mark,
I will report back on the DT2 just to satisfy curiosity of all.
My thought was same as your's, could be 24mm. We'll see, but will remain a mystery until I
get into it.
I cann't see that good either. 1 mm on dia. , pretty small.........

Here's a copy of Parts Bulletin 378 about the change:

So it's mentioned that this new dia. is identical to MX360.
Do you think RT2 & 3 are same as MX360 ? Would make sense.
I looked at a yamaha parts fiche that lists that dia. at 24mm.
I don't own any RT's but am curious.
As mentioned previous, measure before ordering your parts.

I'm not as familiar with the 250/360 but I'm pretty sure it's been mentioned here that the RT pin is 25mm

Gosh, I think these senior moments are getting the best of me lately.
Of course it would have been mentioned on this site at one point or another.
I should try and use the search engine a little more, might answer some questions before there asked.
Have a Great day,
Steve F

Steve, My comment was just to clarify that I don't know the bigger bikes nearly as well as others here...  my answer was based on what I remember reading and not actual personal experience with the 360 crank. 

Shared parts lists and other Yamaha Literature makes this stuff confusing. 

My RT2 parts book list the crank pin as 275 prefix , which is the same pin in the updated parts list for the DT2/3 .
So it will have been 25 mm from day one .

On the early DT1 250's up thru 71 and the RT1 engines up thru the 73 RT3 i have worked on all had or use the same rod. Prefix 214. The 250's had a 24mm pin/bearing and the 360's had the 25mm pin/bearings. Rod prefix 431 can also be used as it has the same dimensions as the 214 rod but has larger cut outs for better oiling of the lower bearing. YZ rod.

At first i assume the RT3 engines i built (long time ago) had a 24mm lower pin/bearing  because of the rod being the same as the DT1's but after going back looking at my records and pictures of the RT360's engines i did rebuilds on i can clearly see they all have 25mm lower pins/bearings in them. 



The MX360 shows it uses the same rod with a 25mm lower pin/bearing which is the same as the RT1 360's  The 74 DT360A uses the same rod too. 

I had 2 new NOS rod kits for sale not long ago. I had both a 24mm and the 25mm set up with the 214 rods. Finally sold both of them on Facebook to one guy. 

 I am sure they changed the rods "only on 72-73 DT250's" not the 360's when they updated to reed valve. Pretty sure this is rod prefix 510 or if it's the original crank/rod has 311 on the rod. They started using the 25mm lower pin/bearing at that time on the DT250's most likely to keep it all simple. 

But as mentioned here already, its best to disassemble and take measurements before ordering parts. 

Bikebuzzard on e-bay has all the different combo's you need if you can't find NOS. 

214 / 431 rods are 130mm. 
311 / 510 rods are 125mm