Are 2.6s the same height / outer diameter as 2.8s or are they smaller?
Bit smaller.
When you pick a 27.5+ frame and you put a 2.6 inch tyre in (as I did) you end up with a biter lower BB.
Could have picked 170 mm cranks - but stayed with 175 mm. No remorse.
Nope they're smaller, although once they compress the difference is less so. This is pretty good as a guide - http://www.bikecalc.com/wheel_size_math
You can work out the missing intervals by averaging etc. There's no magic to it. You'll find though that at the same lower pressure (Say 15PSI) a 2.8 will be closer to its theoretical size than say a 2.6 as the 2.6 is sturdier as a rule, and so stretches less at lower pressures, but the opposite is then true under load.
For the same make/model of tyre then yes a 2.6 will be smaller than a 2.8. For different makes and models not necessarily.