If you want to use 295 or 290mm brake discs, you will have to space the brake caliper to be able to actually fit the caliper on the car. If you would not be using these, the discs will touch the calipers with possibily catastrophic failure as a result.

For the Elise S1 the standard brake disc diameter is 282mm
When using 295mm, you need to space 295-282 = 13mm
You need to divide by two as we're looking at a circle diameter, so you need 6.5mm

We do offer more sizes as spacing on an Elise S1 might be slightly tricky with the 15" front wheels.
Our 295 discs come with a set of four different spacers.

The Elise S2 has 288mm discs as standard.
When using a 295 disc, you need to space 295-288 = 7mm
You need to divide by two again, so you're looking at 3.5mm