Hi Rob,
All depends on what Insulation you want to go for and what U Value you require.
As Dan says 90mm is the most common due to the U Value it will give you.
Is the cavity filled or not?
A 90mm EPS system onto a wall with a filled cavity will give you a U Value of 0.22, a wall with an unfilled cavity will give you a U Value of 0.29
If you need anymore information please let me know