Why not a 2826 or for that matter a 2820? A 3548 is 4oz heavier. Both of those sizes thrive on a 4s in the 2200-2600 range.
A 3515 is about 1/2 the weight of a 3548. I have put a 3520/12 on a .35 size ship and I won't do it again. The 28xx are more than enough.
Ken
It's a different measuring point or nomenclature for the same thing. Internal stator measurements vs the external rotating "Can" measurements.
You just need to look twice at this stuff to see which measuring convention a given brand is using. Most motors from brand to brand will fall within a general weight range, and an outside "can" diamter for any given size, so if something seems strange, check the weight and compare against similar motors.
Here's some examples of cross reference sizing using common motors:
35mm rotating "Can" O.D. = 28mm internal Stator.
- Cobra 2808 = Turnigy 3530 = Park 480
- Cobra 2814 = Turnigy 3536 = Power 10
- Cobra 2820 = Turnigy 3542 = Power 15
- Cobra 2826 = Turnigy 3548 = Power 25
42mm rotating "Can" O.D. = 35mm internal Stator.
- Cobra 3520 = Turnigy 4250 = Power 32 = 35mm Stator