I don't have the miniatures handy to measure, but I think cache is suggesting something around 18-20mm in length, while I am suggesting more like 25-27.
One thing to remember is that once the money is paid to B-3, this is going to the QCC where they will decide on the dimensions. I'm posting my opinion with as much data as I can to justify it. Whether or not they listen is up to them.
That being said...
The 5 ton Savannah Master is 7mm wide x 12mm long.
The 20 ton Blizzard (with 6 tons of cargo space for up to 56 troopers) is 15mm wide x 25mm long without the spoiler.
I think 25-27mm long is way out of the ballpark. I don't think we should be looking to set a new scale standard with these. They should fit with existing vehicles. If the largest Maxim infantry variant (crew + 81 troopers) were sized properly it would be almost the size of the Leviathan warship. That just doesn't work.
You don't need to add a lot of size just to get 7 troopers on board. Take the real-world M1117 Guardian for example: It's listed at 3 crew and 1 passenger with an APC variant having 3 crew and 8 troopers. The standard length (according to the wikipedia) is 237 inches and the APC variant adds 24 inches to the length for the addition of 7 troops. That's only a 10% increase. The percentage size increase for larger numbers of troopers grows considerably.
I'm suggesting the wheeled and tracked APCs should be 12mm wide x 16mm long and the hover APC should be 13mm wide x 16mm long. Also, B-3, I have high resolution images of each of the APCs taken out of TRO: 3039 that I can email to you. They're too big to post.