I just did some quilting with my walking foot and used a 2.5 stitch length (my default stitch length setting is 2.0, but I though that was way too short).

Millimeters? And I think 2.5 is too long for piecing too. I usually go down to 1.8 for piecing. It's a pain if I have to rip, but I make quilts with lots and lots of little pieces and I don't want any of the seams pulling apart before they get crossed by other seams. I set it to 3 when straight-line quilting.

Piecing is 2.5 and quilting is anywhere from 3 to 3.5 depending upon the loft of the batting. For low loft, I prefer 3 and for medium to high I find that it is visually nicer at 3.5.

I like using 2.5 on my Viking

Same here, although for FMQ I often set the stitch length at zero since I'm moving the fabric and determining the stitch length.

I usually use 2.5-2.8 unless I want thread to be more obvious like in shadow quilting, etc., then I use 3.5-4.

I like 3.5

On my Janome the default in Mode 1 is 2.2, but I recently discovered that the default in Mode 2 is 2.5 (handy). However, I sometimes use a longer stitch for quilting, say 3, and possibly a shorter stitch for piecing.

I piece with 2.0 on my Janome 6600 and quilt with the serpentine stitch, or FMQ with a stitch regulator, so the stitch is dependent on my hand movement.

When using my Janome 6500- I set the stitch lenght to 5.0 for SID. Sometimes when doing cross hatching on large blocks I set a very narrow zig zag about 1.5 then the length at 3. Again it depends on the denisty of the batting.