The main Trijicon RMR red dots seem to be 3.5 moa and 6 moa (from memory). For those of you who are using them, which do you prefer? I have seen the larger but not the smaller. Is the smaller easy to pick up?

These days Trijicon quotes 3.25 and 6.5 MOA sized dots. I've used both on pistols and prefer 6.5 myself. The 6.5 dot size better mimics the size of the dot on the front sight. Yes I can crank up the intensity of the 3.25 to make it seem like a larger dot, but most of that (un)focused light winds up being misshapened, off-centered blob of glare courtesy of my astigmatism. Red dots work better for me at lower levels that are "just appropriate enough" for prevailing ambient lighting conditions.

As above, I prefer (and use) the RMR's larger MOA dot on my G19. Because my pistol shooting is at distances less than 25 yd, there is no issue with the dot obscuring the POA and the larger dot is easier to see/quicker for the eye to pick up.

I agree with the 6.5 on handguns - using them on a G19 and G26.

The largest dot I use is 4 MOA so I like the 3.5 in rmr's.

I like smaller dots. All my RMR’s are 3.5/3.25.

I like the 3.5 on the range but for a defense or carry gun I’d go 6.5

Have both, use both, like both, prefer the 3.5.

I use the 6.5 on my G17 and a 12.9 triangle on my PCC HD gun.

Thanks for the input. I will be using it on an STI Costa host (replacing the Vortex viper). Most of my shooting is at indoor ranges and less than 25 yards. In that case I am leaning toward the larger dot.

My experience (now about 25K rounds in various shooting disciplines) is that it doesn't matter a whole lot at shorter ranges. When I want speed I go with an RM07 (my carry gun for example), when I want accuracy I use the RM06 (my current competition gun).
For a range gun as you describe you will be happier with the RM06.