Is there a standard size tire to run on a rear skinny? I have 24's and 26's just wondering what to mount. No one around here has skinnies so not much to go on.

I'm not sure what you mean by tire size. I know Bisi used to run 17x2.5 on the rear of his drag Insight.

I mean tire height, I have 23" slicks on the front for example. Using a 15 x 4 rim on the back, just wondering if 24" or 26" is used more commonly.

By no means am I a pro in this aspect but wouldn't a smaller diameter tire be better? In theory it would save on unsprung weight.

No, i think you want a taller tire with a stiffer rear suspension. It will help keep the weight on the front to improve traction.

If you have an adjustable suspension. This question can be basic or difficult depending on the details.

Yep, I have adjustable suspension and it is set at its stiffest setting, and raised to the max. I creeped the "Sheepybuilt" sfwd drag integra and they run a 25" tall tire on the back. So I don't believe I will need a 26" tall tire. But hey that's why they call it test and tune. I will try em both and see if a 2 inch variance affects the 60 ft or mph on the big end.

In theory, assuming a perfect inflexible circle, a circle will touch a line at exactly 1 point, but since tires are not inflexible, the contact area is flattened a bit. a larger outside diameter will touch a bit more at any given time. This will actually add to rolling resistance. What really matters for the flexible side is the tire pressure itself. when you reduce the pressure, you flatten the tire out alot more than adding a bit of outside diameter does. here's an epic paint drawing to help illustrate my point.... So, in theory, the least resistance tire will be the smallest one at the highest pressure. but then stability becomes the factor