The only real solution to this problem is what Carl said and go to dealers with a tape measure, it’s something I have to do every 3 years to size up luggage space with all the seats in use along with the aperture of the hatch door. Plus boot measurement are always given in litres and I for one do not need to know how many 2 litre milk cartons I can get in the boot.
Getting measurements sat in front of a PC is very hit and miss and you can’t rely on the measurements on Motability site as there often older models and even the manufactures sites themselves can have incorrect information. You could of course ask on this forum if anyone has the model in mind as where very helpful like that. ?
Tape measure and note book is the only way to be sure but agree its one hell of a chore and can make looking for suitable cars a nightmare.