I would think they would fit, but I know very little about fitment issues.
Box is at stock height

Rota Slipstreams:
4 x 100
15 x 6.5
+40 mm offset

Stock wheels:
4 x 100
15 x 5.5
+38 mm offset

How do I pick tires for this to get the same overall diameter?

 

Yes they should fit. You might want to run some spacers, to put the wheels closer to the edge of the fender. The +40 will move the tire closer to the car by 2mm compared to the +38. Do they come in any other offsets?

Edit: You could run stock tires seeing its still a 15" Wheel.