If you can't find someone who knows the actual dimensions, you might try and guesstimate based off the pictures and other "in scale" objects that are shown
For example- the doorway, fuel drum, narrow sidewalk and width of the road in this tends to make me think that the roadway might just be large enough to fit your two kits with some possible modification to the rubble piles already included. The road looks wide enough and long enough to support maybe a Tiger I tank at best.
I’d think if you put the flak in the larger building and the Hortch on the street, you’d have plenty of room to add figures, more rubble, etc. But I can’t be certain because, like the problem you face, I don’t know the actual dimensions. I usually go into looking at kit bases this size with the mindset that they’re designed to fit a medium to large Panzer tank automatically, so this is the size base you might want to look for- for a large vehicle/flak dio- I think you should trace out a street size that’s big enough to nicely fit the width of a Panzer IV or a Tiger and assume that most prefab kits roadways will be about that size, then based on the general length of road you’d need for your vehicle/flak, go from there. But that’s just my opinion.
Use other kits “in scale objects” that are included on the box art to try and guesstimate for yourself. Some places like Squadron.com give you a brief description of the kit which sometimes could include, if not dimensions, a general guideline of what one might be able to fit on the base.
Hope that helps if no one chimes in about base dimensions.