London Great/blue tits 2019

Right amanda here is some of the measurements, The base is 11cm x 11cm, the "ramp wall" is 26cm and other wall on the base is 15cm. 
In the side box the nest does go abit up the "ramp wall" so when the birds nest build the base becomes more like 16cm plus due to them using some of the ramp wall if that makes sense.
look at all the pictures on gardenature website here - 
gardenature.co.uk/product/bird-box-camera-system-side-view-ultra-high-resolution will give you a better look at the box so you have rough idea, If you need anymore measurements let me know got the box handy anyway.