If your cage is probably the standard 113mm that most suppliers use I'd say go for a 103mm
Best to measure from where you want the front of the cage to sit and back to the shallowest place the cage is likely to hit on.
You may even be able to cut notches out of the back of your current one with tin snips to allow the cage to sit in correctly.
I was under the impression that my current cage wouldn't fit my headunit due the the cage being double din and the headunit being single din, I was going to buy that fascia you linked previously and then get a new cage. If the cage is compatible with this new fascia then surely the headunit wouldn't be, the screws/slots on the cage won't line up with the headunit will it?
Apologies if this is hassle but I really don't want to bodge it or have a possibility of something going wrong in the future.