900 x 900 is a fairly common standard but for example we have an 800 x 800 carousel option - whereas we may have about 10 or 15 900 x 900 corner units so your choice will be limited.
I would guess if we can do an 800 unit, you should find some of the other kitchen companies have a similiar option available to you
hope this helps
Regards
barry
Remember that you don't have to use a carousel in a corner. Our corner units,due to be fitted on Monday, go into a bay and have bi-fold doors which allow easy access right into the cupboard.