I have the flowhive 2+.
Does anyone know the dimensions of the (inner cover) directly below the roof?
It will be the same dimensions as the outside of the boxes.
That does not answer my question.
Hello and welcome to the Flow forum 
I would suggest emailing the tech support team at Flow, as most of us here are beekeepers, and not experts on the exact dimensions of the Flow hive. You can reach them at info@honeyflow.com, and if you send your order number, they will be able to give more precise information about the model of Flow hive that you purchased. 
They are very helpful and usually respond quite quickly. Make sure to check your junk mail folder if you haven’t heard back within a couple of days. I will also tag @KieranPI, in case he can give you a faster answer
Sorry, I thought because you had a flowhive 2+, you could simply measure either the brood, or honey super. The only other measurement would be the depth, which is 20mm.
I’m not sure if the 2+ only comes in one size. The other issue would be whether you have a 6 or 7 frame flow super, which will mean an 8 or 10 frame brood box Therefore the inner cover comes in 2 sizes.