Just won some s13 calipers on ebay without pics...winning bid was $1 so i was pretty happy.
wanted to know if the 250mm and the 280mm calipers have any obvious diff in appearence. Apart from being slightly larger.

280mm's are twin spot and 250's are single

I'd like to correct you there sorry, you might be thinking about S14 calipers. The 280mm is proportionally lager than the 250mm and has squarish brackets.
S13 250mm vs S13 280mm


S13 280mm in 14x6 wheel

The 250mm rotor and 280mm rotor both use a caliper that looks nearly the same, but what are the caliper mounting bolt hole spacing?

They use the same brake pads, and the caliper looks the same.

I've tracked a couple of late 94 and 95 model SR20 S13 180SX's that came with the 280mm brakes, with various Nissan calipers and discs.

Both S13 250 and 280mm brakes are single piston slider calipers.

There is a twin piston slider with both pistons on the inside of the caliper body. The first 180SX I bought was a street/drift car from Japan. It had these calipers on it. Buest guess the importer could come up with is they are from a Cefiro.

S14s use a cast iron Sumitomo 4spot caliper on the same 280mm disc. I upgraded to them not long after.

And later upgraded again to a set of R32 GTR cast alloy Sumitomo 4spots and 296mm discs.

I still have the twin piston Cefiro and 4spot S14 calipers lying around. I also have a set of 250mm S13 brakes for the 120Y. They all have the same bolt spacing.

The caliper bodies themselves accomodate the larger diameter discs, and they can be bolted onto any S13 upright. So any of them could be used with the S13 caliper adapters on 120Y struts.

To answer the original question, you can tell the 250 and 280mm calipers apart by how close the mounting holes are to the piston, but only once you're familiar with them. The casting for the piston just dips down in between the two bolt holes on the 250mm calipers, whereas it doesn't on the 280s. You might be able to find pics on the net to see the difference.

Bolt spacing is 100mm

Some S13 calipers have a larger diameter piston.

Yes, the CA/SR20DE piston uses a 48 mm piston
The 280mm rotor uses a caliper with 53 mm piston. And the bolt spacing for both calipers seems to be exactly the same.

so will the 280mm calipers work with the 250mm rotors??

No. the 280mm caliper has the pads further away from the mounting holes to acccomodate the larger diameter rotor.