Has anyone mounted a Trijicon SRO onto a P226 or P229 ?? I’m thinking about doing that (if it’s possible) and also wondering what MOA dot size to buy. I’d use it for mostly <20 yard tactical pistol shoots.

(I’ve searched the archives with no results)

Dot size is really personal preference. Up close generally a larger size is preferred because wobble is not amplified and precision is not paramount.

I have shot a lot with both 6.5 and 3.25 MOA RMRs and overall I prefer the 3.25 for my match pistol which is used primarily for targets from about 10 to about 50 yards primarily. Of course 2 and even 1MOA dots are now available. If you have a lot of experience shooting handgun dots, a 1MOA can be very versatile. But for those with less experience the smaller the dot the more they can appear to wobble or accentuate the natural tremor of the grip. It’s unnerving. If you can, check out a 5-7MOA dot on a handgun and see what you think.

I have a 5 MOA SRO and multiple 3.25 MOA RMRs. While shooting I don't notice a huge difference between the two. This could be because until I bought the SRO the RMR was the only optic I had and I didn't have much trouble getting used to what is said to be a relatively small dot size.

In addition, I had my P229 9mm milled not long ago and put an RMR on it. The cut leaves room for suppressor height sights (which I have not installed yet) but it looks to me that if I put the SRO on it the front edge will be over the ejection port (which is not the case with the RMR) and I think that may cause problems. If the cut is moved farther back and eliminates the rear sight dovetail then I think there is plenty of room for an SRO. It's unlikely I'll install the SRO on the P229. If I had to keep only one optic I would keep the RMR. For match shooting the window is a little small but I like the smaller dot in general and if the 'presentation' is good (and mine isn't that great yet) the smaller window isn't that big of a deal in my opinion.

If you have an E2 P229 9mm you could also consider buying a slide from Sig with a Romeo 1 Pro installed or a 'pro cut' slide which I think will mount RMR-footprint optics but I'm not sure about that.

On dot size I have tested it extensively and find that the RMR 3.25 is just about the best optimum for what I do which is just a bit longer ranged than you. But I certainly had no issues (and actually was a bit faster at close ranges) running the bigger (6.5) RMR dot. Of the two SRO choices I would pick 5 myself.
I bought a couple of SRO's as experiments. My experience has not been good when slide mounted on a pistol. The shape change means that the front element is simply in the wrong place on a gun milled for an RMR with respect to the ejection port. I think several people are in the process (or have) made adjustments on the mounted location on some guns to deal with it (the M17 comes to mind, but that's a plate). I'd be sure to discuss it with whoever mills your slide.